I never cease to be amazed by the number of folks who have a critical event coming up in a few days and think NOW is a good time to mess around with a working PC -- and, like you, they always do it (1) without a working backup, and (2) no way to restore their PC to a working state.
If you have a bootable Vista DVD, or a Vista Repair CD, what might work is booting from it, choosing command mode, and then enter "sfc /scannow".
That will search for missing/damaged system files and will attempt to repair them.
If you don't have such a CD/DVD, and you have access to a working PC with a CD burner, then you can go to the site linked below, download and burn a Vista Repair CD -- and use that: (but it will COST you)
http://neosmart.net/blog/2009/window...-repair-discs/