Windows Updates don't work

If Windows Updates download but fail to install, Microsoft knowledgebase 943144 provides a fix:

http://support.microsoft.com/kb/943144

Cleanest option is to re-register wups2.dll in Windows per KB943144.

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To register the Wups2.dll file in Windows, follow these steps:
# Stop the Automatic Updates service. To do this, follow these steps:
a. Click Start, click Run, type cmd, and then click OK.
b. At the command prompt, type the following command, and then press ENTER:
net stop wuauserv
# Register the Wups2.dll file. To do this, follow these steps:
a. At the command prompt, type the following command, and then press ENTER:
regsvr32 windir\system32\wups2.dll
Note For a computer that is running Windows XP Professional x64 Edition, type the following command, and then press ENTER:
regsvr32 windir\syswow64\wups2.dll
b. Click OK on each verification message that you receive.
# Start the Automatic Updates service. To do this, type the following command at the command prompt, and then press ENTER:
net start wuauserv
# Exit the command prompt. To do this type exit, and then press ENTER.
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Per Hubert’s original recommendation, presumably also based on KB943144:
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There is a standalone installer for Windows Update, located for download at

http://download.windowsupdate.com/v7/windowsupdate/redist/standalone/WindowsUpdateAgent30-x86.exe

This is a nice tool if you run into trouble with Windows Update failing.