You should have a copy of your install disk really

but I would download an iso if you haven't.
A good idea when building systems is to take a system backup using the built in software before you install stuff then you can restore the base image at any time. All you need to do then is keep a recent backup of your registry hive ( do a system state backup which creates a pretty small file ) and if the O/S files corrupt you can restore your base image and then restore your registry hive followed by a system restore point to get you back to a fairly recent state of working.
The fact you re-install over your original O/S means that only the registry will be altered ( new install no program entries ) but the system state will restore your registry and most of your installed apps should then work again.
You will still get a few problems but most will be solved by either an app re-install or a few dlls being re-registered.
I would be inclined to check your drive for problems tho as vista doesn't corrupt files very often on its own
