i would at least try a repair install before i formatted, etc.
you put in the cd and go through it like you're doing a fresh install, (do not go to the repair console, thats not what you want). eventually you'll have to select the partition, if you select the one that its already installed on, it will ask if you want to repair install, do that, it might fix the problem. it doesnt touch any of your programs or files, only system files, so you dont have to back stuff up. if that doesnt help, then format it is.