For whatever reason, the installation of Windows Service Pack 3 breaks the windows update service. You won't see any direct symptoms of this other than the obvious; every time you try to install Windows Updates, they will fail seemingly without reason. Simply giving the error message "The following updates were not installed".
The steps for fixing this problem are lined out in this link (ignore the page title, the important thing is the registering of the DLL file, not what broke it in the first place).
Since I'm tired of having to hunt down the locations of the DLL file and whatnot, I'm simply going to put the instructions here (copied from http://support.microsoft.com/kb/943144)
1. 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
2. Register the Wups2.dll file. To do this, follow these steps:
a. At the command prompt, type the following command, and then press ENTER:
Note For a computer that is running Windows XP Professional x64 Edition, type the
following command, and then press ENTER:
b. Click OK on each verification message that you receive.
3. Start the Automatic Updates service. To do this, type the following command at the command prompt, and then press ENTER: net start wuauserv
4. Exit the command prompt. To do this type exit, and then press ENTER.
These steps have worked pretty well for me thus far, and it's not that complicated of a fix, but wouldn't it just be easier if there was something written into the service pack itself to re-register the DLL?? I'll often stick up for Microsoft against my better judgement, but this is pretty ridiculous.