Some updates were not installed...

I get this error frequently. Windows Update (WU) will find all available updates for my machine, download them, and attempt to install them, but reports the error: Some updates were not installed. A fix I have stumbled across that has worked every time is to force reinstall WU. The steps are fairly simple.

• Download a WU executable from Microsoft.
• Save the executable to your desktop.
• Open up command prompt (Start > Run > cmd) and cd (change directory) to your desktop.
• cd Desktop (Assuming you are at C:\Documents and Settings\)
• Next type the name of the executable (windowsupdateagent30-x86.exe) followed by the switch /wuforce
• WindowsUpdateAgent30-x86.exe /wuforce
  
• Note: When typing the name of the executable, you can enter a bit of text and then press tab to autocomplete your file name. Example typing windows then pressing tab would give me windowsupdateagent30-x86.exe

That's it. Now just go to Windows Update and it will require you to reinstall WU. After that you should be able to download and install properly. Hope this helps.