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. 
 
 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 . - — 
 Per Hubert’s original recommendation, presumably also based on KB943144: — 
 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.