First thing to try is to reset the Windows Update mechanism on your PC.
To do this, follow these steps (am writing from memory of XP now i'm running W7, but it's likely to be very similar for all Wintel OSs at present):
1) stop the Automatic Updates service (found in Control Panel/Services)
2) navigate to %systemroot%\softwaredistribution
3) delete everything within that folder
4) start the Automatic Updates service
Then either wait for an update to occur, or go to Microsoft Update (or Windows Update if you haven't already said yes to Microsoft Update) which is found in your start menu, and get it to scan your PC.
HTH
Mike.