Moved this to where it belongs.
Alvin is correct. while you could skate with XP changing mobos (sometimes, if you knew what you were doing), Vista is not as forgiving. You will have to reinstall.
If you have data you can't afford to lose, you need to put your hard drive in another computer as a slave and copy off what you need to save. That, or use a different hard drive to install Vista on, and use this drive as a slave. This is all assuming you don't have separate data partitions, but in looking at you problem I seriously doubt that you would.