Yay Vista! Gotta love how Vista continually amazes me with all the file corruptions it causes. Anyway, it sounds like you have a corrupt boot sector, probably caused by the recent update. If you have access to another computer, try visiting www.freewarehome.com then go to System Utilities < Disks, HDD and storage < Boot disks.
If you know what you're doing, or feel confident in your abilities, download one of those programs and load it onto a USB drive. Then use the USB drive on your computer w/ the boot issue and attempt to rebuild the boot sector of the drive. I've done this a few times, and it can work wonders. It can also cause you to make the drive unbootable. So please, if you don't feel comfortable rebuilding the boot sector, don't mess with it.