Hi Moonfrost,
Well, that's strange. I had similar issue once, don't remember what I did to get that, but I think I was playing with registry cleaner or software uninstaller which corrupted registry. I repaired my system with OS backup.
I think, you can try to repair operating system using this
tutorial how to repair Vista, to solve this problem. If this won't help, the reformating your drive will be the best thing to fix it, I suppose.
Hope it helps. Cheers...