If Windows XP, Vista or 7 do not start at all or reboot or stop responding while starting, the first thing to try is the Last Known Good Configuration. This one is especially useful when the problem appears right after some system crash, software or driver installation or upgrade, or when you edit the Registry manually or change advanced system settings.
Windows always monitors if it starts and works correctly. If everything is fine, Windows saves its good configuration to a special hidden folder during shutdown (I will not tell you its name and location you curious minds! ).
With some luck, this troubleshooting option can even get around the dreaded "C:\windows\system32\config\system is missing or corrupt" and the "Inaccessible boot device" 0x0000007B errors.
If Windows starts properly after using this option, you must verify that there are no events with ID 7 concerning your PC's hard drive in Event Viewer's System log. If there are, the drive is about to fail and you must replace it as soon as possible.
The Last Known Good Configuration option is available in Windows XP, Vista and 7 only. In Windows 8, 8.1 and 10, try using Safe Mode instead.
To get access to restoring the Last Known Good Configuration, you will have to press the F8 key on your keyboard after the big logo or black screen with white texts appears.
On some computers, you will see a boot selection after pressing F8, cancel that by pressing Esc key on your keyboard and then press the F8 key again.
First, you might see an operating system selection screen. Select the correct one with arrow keys and press Enter key. If you already pressed F8 repeatedly, this screen might not appear.
You will see the screen as follows. Use the Arrow Down key on your keyboard to move to Last Known Good Configuration (your most recent settings that worked) (in Windows XP) or Last Known Good Configuration (advanced). Press ENTER key to start Windows with Last Known Good Configuration.
If Windows starts and works normally now, you've resolved the problem. If not, continue reading about the Safe Mode option.