zkus tohle

1- Run this KB (it will act like it is installing an update but it is actually running a check on the system)
http://support.microsoft.com/kb/947821
Then after it runs, reboot and run Windows update. But when you run Windows Update make sure that only Vista SP1 is attempting to install and not any other updates.

2- If that didn't help then run a System File Checker. Below is a link with instructions. When it finishes it will state if it found any problems and if it fixed them or not. If it fixed them, attempt to update again. If it could not fix them then time for a reinstall(post back if this is the case and someone can give help with that).

3- If #2 did not find problems OR fixed problems but the update failed then run a checkdisk. Here's a link on how to do that: http://www.windows-help-central.com/windows-vista- chkdsk.html
After running chkdsk attempt the update again. Post back with the results.