If you know the update that caused the problem, then simply uninstall it, reboot, and that should hopefully return things back to normal. Then post the specific update name and KB number so we can research the problem more thoroughly. Depending on the problem, you may need to try to boot into Safe Mode to do this if you can't boot into Normal Mode (I'm not clear whether or not that's a problem).
If that doesn't work, do you know when this problem began (I assume around when the update was installed)? Try a System Restore to a point in time BEFORE the problem began - and try to give it a little extra leeway. Here's the procedure: http://www.vistax64.com/tutorials/76905-system-restore-how.html. Be sure to check the box to show more than 5 days of restore points. If the first attempt fails, then try an earlier point or two. NOTE: You will have to re-install any software and updates you installed between now and the restore point, but you can use Windows Update for the updates. Use the recovery disk if the system prompt doesn’t work. The recovery disk works a bit different from the above procedures but if you follow the prompts from the System Restore menu option with the above information you should be able to restore with no problems.
If you don't have a disk, you can make a bootable Recovery Disk using http://neosmart.net/blog/2008/windows-vista-recovery-disc-download/ along with burning software like: http://www.snapfiles.com/get/active-isoburner.html and, of course, a blank CD (perhaps made on a different computer or perhaps in safe mode with networking if that is working). To boot to the CD you may need to change the BIOS to make the CD-drive first in the boot sequence. To do that, wait for the screen that tells you the F key to push to access the boot menu or boot setup. Push it quickly. Make the changes, save your work, and exit. Put the CD in the drive and reboot. When prompted, push any key to boot from the CD.