Man, it's an easy fix. Grab your trusty XP disc, pop the sucker in. Reboot and boot from the cd. Go ahead and when it asks you to install windows, say repair. What this will do is copy all needed files for XP from the CD onto the existing directory. This replaces all missing / changed files. Then bam, you've got all your setting and data still there. Although, you will probably have to update again, but I think that is a decent trade off to reformatting and having to update anyways.
Also, where'd you get Vista? It's on sale?