Search for "Windows Update". Blast it all. Don't forget to follow the class IDs if necessary.
I haven't done it recently. I know that when Windows Update is done, it's done. You can't fix it in any sane way (it's Windows).
Run a regvac afterwards to gather up stray keys.
Remember you'll have to back up to whatever SP you were on, then start applying updates again.
Remember to turn off "System Restore" on Windows XP.