The "joys" of Windows Update not working in Windows XP, Vista, 7, 8 and 8.1 are numerous. This might end with not receiving or not being able to install updates, repeated installation of same updates, or in worse cases, endless reboot cycles in Windows Vista, 7 or 8/8.1.
First, check if you have enough free disk space (at least 1 gigabyte) on your hard drive. If not, read the Remove temporary files article on how to get rid of unnecessary files with CCleaner.
Second, verify that time and date are correct on your PC - a date in the past or the future prevents both Windows Update and Genuine Windows Validation tool from working properly.
Luckily there are two small programs for quickly fixing Windows Update problems or completely reinstalling it in Windows XP, Vista, 7, 8 and 8.1.
You can also run both tools in Safe Mode with Networking, although the tools might then tell you that problems were not fixed.
Windows XP users, if you experience serious and prolonged (several hours) slowdowns while Windows Update is searching for updates, and Task Manager shows that svchost.exe is constantly using 99-100% of CPU resources, see this tutorial at www.winhelp.info. This applies to brand new (aka clean) installs of Windows XP Service Pack 3, and to those who have long turned off automatic updates and now want to apply latest patches.
Windows 8 and 8.1 built-in troubleshooter
Windows 8/ 8.1 users can open Settings search using keyboard shortcut Windows Key+W (or by swiping in from the right edge of screen and tapping Search icon). Type troubleshoot and click or tap Troubleshooting.
Then click Fix problems with Windows Update in the System and Security section.
Click Advanced, verify that Apply repairs automatically is enabled and click the Run as administrator link.
You might have to restart your computer for changes to take effect.
Option 1: applying quick fixes to Windows Update
This option now works in all supported Windows versions (from XP to 8.1).
First thing to try is to open Microsoft's How do I reset Windows Update components page.
If the following dialog pops up on the web page, click Run now.
If the pop-up does not appear, expand the appropriate section (Windows 8 and Windows 7, also applies to Windows 8.1; or Windows Vista and Windows XP) and click the run now button.
Click Detect problems and apply the fixes for me (Recommended).
The Fix It tool will then create a System Restore Point, download Windows Update components and run fixes. This usually takes a few minutes only. Click Next after the process is complete.
Please note that in Safe Mode the Fix status column will always state "Not fixed" because Automatic Updates service cannot be started.
After this, you can test if Windows Update works correctly. Closing the Fix It tool is optional.
If you want to give feedback to Microsoft, choose an option in Did this fix your problem screen and click Next.
Finally, click Close in the Thank you screen.
This option is available for Windows XP, Vista and 7 only.
If the first option did not help, Microsoft also has the very powerful Fix It Tool 50202 that completely reinstalls Windows Update components. This one is especially helpful in cases where Windows Update is not able to check for updates (for example, error 800700C1) or installs the same updates all over again (several cases with infinite loops of .NET Framework updates).
Because Microsoft just loves to move the tool randomly around (and its site search is not helpful either), I now provide direct download link to Fix It Tool 50202.
Please save the file, do not run it automatically!
After downloading is complete, run the program (named MicrosoftFixit50202.msi).
Click I Agree in the License Terms page and then click Next.
As you can see, the Run aggressive options (not recommended) check box on Additional options screen is unchecked. I strongly suggest to execute the repairs without enabling aggressive options for the first time - this will retain all Windows Update and Microsoft Update settings.
In Windows Vista and 7, User Account Control will appear. Click Continue or Yes.
The tool will then create a System Restore point and run the fixes. Some files might be downloaded during the process.
After the program has done its job, click Close.
A prompt to restart your computer appears. Save open documents and close all open programs. Then click Yes.
Verify that Automatic Updates, Windows Update or Microsoft Update works. If not, re-run the Fix It Tool 50202, but tick the Run aggressive options (not recommended) check box this time.
After using aggressive options, you must configure Windows Update again.