Symptom: Neither Windows Update or Microsoft Update would complete. The machine in question in this instance was a vanilla P4 somethingorother, running XP pro. Before finding this solution I tried a bunch of things, including an install of Service Pack 3.
You can read the full thread on the techarena forums. I found it via a google search, I'm not familiar with this forum in general. The bit that I needed is from post 3 in the thread:
Start regedit.exe and delete the following key (contains the WU
Do a reboot now!
Then run this command line (from a command prompt of from Start/Run, it
installs the AutoUpdate service (line will wrap in the newsreader!):
%SystemRoot%\System32\rundll32.exe setupapi,InstallHinfSection DefaultInstall
(it must be a space between DefaultInstall and 132)
If it asks for your OS CD-ROM to get some files, just point it to the
folder %windir%\System32 (%windir% is typically C:\Windows).
Then run the following commands:
Do a reboot again.
Then check if you can start and stop the Automatic Updates service
successfully from services.msc
These steps repaired the problem.
* Caveat: This sort of stuff does change from time to time, but it should still serve as an indication of the sorts of problems you should look for if you have similar symptoms. At the very least, you might include some of the relevant terms in your search to speed things up.