If you own (or can borrow) a Vista installation disc which is the same service pack level as your installed version, I suggest you try a repair. Tutorial here:
How to automatically repair Windows Vista using Startup Repair
It is perfectly legal to borrow a Windows disc just for repair purposes, but if you are prompted to enter a product key at any time, you must enter the product key that's tied to the PC you are repairing. If Windows was factory installed, the product key will be on a sticker attached to the PC (on a laptop it's underneath, on a PC it's on the back panel where all the ports are).