No operating system is bulletproof. Many things can go wrong - sometimes files go messy on a hard drive or an SSD after a power failure, sometimes a bad software or driver installation causes trouble, sometimes user deletes something important, etc.
This section will cover the most annoying problems - Windows will not start or run properly. The remedy may be as simple as pressing a few keyboard keys, or it may take some time or even hours of repairing system files or performing a non-destructive reinstallation.
You can also access troubleshooting tools by booting from Windows installation DVD (applies to Windows Vista and later only).
If you need to boot from a CD/DVD or USB drive, see the Computer boot order article for instructions on how to change the PC boot order.
Always check for error and warning events in Event Viewer, this might help a lot. To identify file system corruption or hard drive problems, look for events with ID 7 and 55 in System log.
In Safe Mode, ignore events such as DCOM "This service cannot be started in Safe Mode" (Event ID 10005) and events with ID 7001. These events happen because Safe Mode starts only essential services to help fighting malware or stop problematic programs from loading.
Windows Vista, 7, 8, 8.1 and 10 users can use Reliability Monitor for troubleshooting applications that crash or stop responding.
All Windows versions have the built-in DirectX Diagnostic Tool for troubleshooting display and audio driver errors - just use the keyboard shortcut Windows Key+R to open the Run dialog, type dxdiag and click OK.
In Windows XP, a prompt for checking WHQL digital signatures appears first, click Yes there.
Open the Display tab and verify that the Notes field contains "No problems found". If there are problems, you must reinstall or update the display driver.
If you experience frequent computer crashes (aka Blue Screen of Death, BSOD), analyze crash dump files with WhoCrashed Home Edition to see which program, driver or hardware component is the cause.
Then try testing your computer's memory with Memtest86+. It doesn't help much when you spend hours fixing your Windows and then a defective memory module ruins the whole thing again. And believe me, defective memory is often the root of all evil when your Windows does not start or acts erratically.
In case Windows is able to start and you can log in successfully, you should try running System File Checker first. This program verifies that files required to run Windows are intact and if necessary, replaces damaged, missing or malware-modified files.
Please note that System File Checker is not able to run properly in Windows Safe Mode.
Windows XP users need the Windows CD that came in the product box or with the computer. You can use friend's CD here, but make sure it matches the version of your Windows XP - do not use Windows XP Home CD-s for repairing Windows XP Professional installations and vice versa. Do I need to say that you cannot use Windows Vista, 7, 8/8.1 or 10 DVDs for repairing Windows XP files?
Windows Vista and later versions have a special folder where required files are cached, so the installation DVD is not usually required.
In Windows XP, open Run dialog using the keyboard shortcut Windows Key+R or by opening the Start menu and clicking Run. Type cmd and click OK.
In Windows Vista, 7 and 10, open Start menu and type cmd into the Search box. Right-click cmd.exe or Command Prompt and select Run as administrator. This will open the so-called elevated command prompt.
In Windows 8 and 8.1, open Start screen and type "cmd" - Search pane opens automatically (you can also open Search pane using the keyboard shortcut Windows Key+Q). Right-click Command Prompt and click Run as administrator.
Windows Vista, 7, 8/8.1 and 10 users will meet the most beloved User Account Control. Click Continue or Yes here.
Now, in the black Command Processor window, type sfc /scannow and press Enter key to run the command.
In Windows XP, a separate Windows File Protection window opens. In a minute or so, another window pops up, asking for Windows XP CD. Insert it now and the latter dialog disappears automatically.
Please note that you do not really need Windows XP Service Pack 3 CD, the original one will do just fine.
In Windows Vista and later, the whole process continues within the same Command Prompt window. The checking and repairing take quite some time in all Windows versions - 30 minutes or more is absolutely normal.
If no missing or damaged files were detected or no restart is required to complete the changes, the Windows File Protection window closes automatically in Windows XP. You can check System log in Event Viewer for entries that have "Windows File Protection" written in the Source column.
In Windows Vista and newer, "Windows Resource Protection did not find any integrity violations" appears. This means that all files were present and unmodified.
In case some damaged or missing files were detected and replaced, the message reads "Windows Resource Protection found corrupt files and successfully repaired them".
If a restart is required for changes to take effect, an additional line "The system file repair changes will take effect after the next reboot" appears.
You can then safely close the Command Prompt window. If necessary, restart your computer and see if Windows works better now.
In case you get an error message "Windows Resource Protection could not perform the requested operation" during the scan or "Windows Resource Protection found corrupt files but was unable to fix some of them" after the scan, this most probably means that permissions of a file do not allow checking or replacing it.
To resolve this, first type cd /D %windir% and press Enter. This will open the folder where Windows is installed. Second, type icacls * /T /Q /C /RESET (mind the spaces!) and press Enter. This will revert file and folder permissions in your Windows folder to their defaults. You will see really many "Access is denied" messages in the process, but this is expected. Please use correct capitalization in the commands!
In extreme cases, resetting permissions requires activating Windows again after the next restart - please find your product key first.
After this, try running sfc /scannow again.
If you still get the same error, use the chkdsk /r command to run an exhaustive disk check. Because Windows is running from the same drive, you will see the "Chkdsk cannot run because the volume is in use by another process" error. Press Y and then press Enter to schedule the disk check for the next restart.
Restart your computer right away, but be aware that the full disk check might take from 30 minutes to several hours to complete. Do not reboot your computer during the test!
After the check has completed and you are back in Windows, launch System File Checker once again.
To see the System File Checker log in Windows Vista and newer (beware, it is a pretty technical and large one!), you need to run Notepad with elevated rights (right-click, Run as administrator), browse to the C:\Windows\Logs\CBS folder and open the CBS.log file. Opening the file normally ends with an "Access Denied" message.
Please do not worry if you do not experience any problems with Windows, but SFC fails to repair all files. If everything works, there is no need to fix it.
Pressing F8 key during boot-time opens the Advanced Boot Options menu for Windows XP, Vista and 7. It offers several troubleshooting options: Last Known Good Configuration, Safe Mode, VGA Mode, etc. Always try Last Known Good Configuration and Safe Mode first if Windows is not able to start properly.
In Windows 8, 8.1 and 10, the F8 key trick is not available by default and the Last Known Good Configuration option has been removed altogether. Accessing the Advanced Boot Options menu requires starting Windows and then rebooting to Advanced startup via the PC Settings app. Alternatively, hold down the Shift key while clicking Restart.
Advanced repair options also become available if Windows 8/8.1 or 10 is not able to start three times in a row, or if Automatic Repair fails.
If really required, you can still temporarily enable the good old F8 key trick in Windows 8 and later. Boot from Windows installation media or Recovery Drive/System Repair Disc and use bcdedit to enable Legacy Boot Mode. Please note that this legacy feature should be used during troubleshooting only - it might cause problems on UEFI (newer version of BIOS) systems.
- If Windows crashed and does not start anymore, try Last Known Good Configuration (in Windows XP, Vista and 7 only) first. If that does not work, try Safe Mode or use System Restore.
- If you changed advanced system settings or edited registry, try Last Known Good Configuration first (only in Windows XP, Vista or 7). If that does not help, try changing the settings back by going to Safe Mode or using System Restore.
- If you installed or updated some software, try uninstalling it in Safe Mode. If the software uses Windows Installer service, enable Clean boot mode or use System Restore.
- If you updated or installed a driver, try Last Known Good Configuration (in Windows XP, Vista and 7 only) first. If that is of no help, try rolling back the driver or removing the new driver in Safe Mode or use System Restore.
- If you added new hardware, such as hard drive, sound card, a memory module (RAM), etc, remove it from your computer and try starting Windows normally. If Windows starts and works fine, read the manual that came with the new hardware or consult the supplier of the hardware. Sometimes you need to take some certain steps before installing new hardware. Not all hardware works with every computer!
- If you added a new video/graphics/display adapter (GPU) or replaced your monitor and Windows does not start or the screen goes blank, try using Enable VGA Mode, aka Enable low-resolution video mode and changing the screen resolution to the minimum or recommended level.
- If you experience frequent blue screens (also known as Blue Screen of Death / BSOD) and automatic restarts, use free WhoCrashed for crash dump analysis. Also, files on your hard disk might be messed up - start Windows in Safe Mode, or use Recovery Console (only in Windows XP) or Repair Your Computer in Windows Vista and 7, Windows 8 and 8.1 or Windows 10 for repairing file system errors.
- If Windows starts or runs properly only in Safe Mode, turn on Clean Boot Mode to find out if a third-party software or driver is causing this.
- If Windows works but is slow and many programs crash, run Memtest86+ first to ensure that memory modules (RAM) are fine. If they produce errors, replace or remove the faulty modules. Task Manager and Resource Monitor also help to identify the processes with the highest CPU and memory usage.
If RAM is working without errors, run a full anti-virus scan.
In case malware is not the problem, use the non-destructive reinstall/in-place upgrade (available in Windows XP, Windows Vista and 7 and Windows 8/8.1 and 10) method that repairs Windows without deleting your programs, files, and settings. In Windows 8 and 8.1, this method is the only solution to the infamous DISM RestoreHealth error 0x800f081f.
Windows 8 and 8.1 also offer the Refresh your PC option that keeps your files and personalization settings but removes programs and apps that were not installed using Windows Store unless you've created a custom recovery image.
Windows 10 has the Reset this PC feature that preserves your files and personal Windows settings but deletes all non-Windows Store programs and apps.
- If nothing else works, you can still recover your files using Puppy Linux (in case hard disk itself is fine).
After you store your important files elsewhere, reinstall Windows XP, Vista or 7.
In Windows 8 and 8.1, you can use the Reset your PC option for this.
Windows 10 users can launch Reset this PC or Recover from a drive for reinstalling Windows.
When nothing helps, grab your last backup (you DO have a backup?)
Sadly, there are times when Windows cannot be recovered to a previous state - if your device's hard drive or SSD fails completely and no data can be recovered. You will have to buy a new drive unless it is covered by warranty. Or maybe some virus or malware deletes most important things, such as SAM (Security Accounts Manager) database or registry so that Windows knows nothing about users or itself anymore.
Please read about backing up Windows and recommended programs.
If Windows detects that it did not shut down correctly the last time, it opens Windows Error Recovery options after you turn on your computer.
If this happens for the first time or if you know that there was a power failure or something else unintentional caused your computer to shut down unexpectedly, you can select Start Windows Normally with arrow keys on your keyboard and press Enter key.
In case this error screen appears repeatedly, you should select Safe Mode instead: this could be just a problem with messy files on the drive and Safe Mode always checks and repairs disk errors before starting.
Here Windows failed to start. If you just added some new hardware to your computer, you should try removing it and starting Windows again to see if it now works fine. If it does, consult the manual of your new hardware to see the installation steps and compatibility for your version of Windows.
If there was a power failure while starting Windows, select Start Windows Normally.
If you haven't changed anything:
- In Windows XP, start Safe Mode and verify Windows is able to start. If it does not start, boot from my Data Recovery CD and run a full disk check.
- In Windows Vista, boot your computer from Windows Vista DVD, select your regional settings and then click Repair your computer. Follow instructions.
- In Windows 7 and later, select Launch Startup Repair (recommended) and follow instructions there.
"NTLDR is missing" (in Windows XP) or "BOOTMGR is missing" (in Windows Vista and newer) means that something or someone has deleted the file and Windows has no idea from which hard disk or partition to start up.
Windows XP users should boot from my Data Recovery CD and run a full disk check. If this does not help, perform a non-destructive reinstall.
Windows Vista and later users should boot from Windows DVD, select their regional settings and then click Repair your computer.
You should run a full virus scan immediately after Windows works again to prevent this situation from repeating itself.
In case Windows repeatedly restarts and you see a blue screen flashing before each restart, you probably have not enough time to read the message. To find out what's causing the behavior, you should disable automatic restart on system failure.
When your computer starts, you might see some full-screen logo or black screen with gray texts such as "AMI", "Intel", "Testing Memory", "Hard disk", etc. Press F8 key on your keyboard repeatedly right after you see such screen disappear. This will open the Windows Advanced Boot Options menu.
Use Arrow Down key to select Disable automatic restart on system failure and press Enter key.
In case of failure, the blue screen will not disappear anymore and you can see the details of the error.
Here is one infamous example of Blue Screen of Death (BSOD) - error code 0x0000007B meaning "Inaccessible boot device" - Windows cannot use the hard disk it is installed on. This usually happens after you change/update/remove drivers for a hard disk controller; or you install a new hard disk controller (IDE, PATA, SATA, SAS or SCSI) card and connect your hard disk to it, but there are no drivers available for the new controller in Windows.
You must press the power or reset button on your computer to continue.
If you're lucky and you only changed or removed drivers, you can achieve success by selecting Last Known Good Configuration from Advanced Boot Options menu (use the F8 key again after you turn your computer back on) the next time.
In case Last Known Good Configuration does not work, Windows XP users should boot from my Data Recovery CD and run a full disk check. If this does not resolve the problem, do a non-destructive reinstallation and install proper drivers (you might need to press the F6 key while Windows setup is loading to add new drivers in case Windows XP has no clue about the disk controller).
Windows Vista users can also boot their PC from Windows DVD and use the Repair Your Computer option.
Windows 7 users should try the Repair Your Computer option from the Advanced Boot Options menu or Launch Startup Repair from the Windows Error Recovery menu.
If you changed or added a disk controller, disconnect the hard drive from the new controller and connect the cable back to motherboard connector (or where it used to be before). Then boot to Windows and install proper drivers for the new controller. Also, consult the manual of the new controller for detailed installation instructions. Then turn off your computer and connect the drive to the new controller and try booting to Windows again.