If Windows Update mysteriously fails to install updates, it may become necessary to completely clear the Windows Update cache. This resets the state of updates, removes downloaded files and requires Windows Update to completely re-analyze your system to figure out what updates are required.

The procedure is non-invasive and can be performed on workstations and servers alike and without downtime.

1. Open an elevated command prompt.
2. Stop the Windows Update service:

   net stop wuauserv

3. Remove the SoftwareDistribution folder, containing the complete cache and downloads:
   From the command prompt:

   cd /d %windir%
   rd /s SoftwareDistribution

   Or, remove the folder through Windows Explorer.
4. Start the service again:

   net start wuauserv

The cache has now been cleared and you can perform a new check for updates. It may take a while for the check to complete as Windows Update has to analyze your system from scratch.

If the above procedure did not work, you may have to completely reset Windows Update components (KB 971058).