Hi. I had a similar problem once. In my case, it was the case of an operating system file that got corrupted. My problem was solved when I repaired (not re-install or re-format) the operating system.
Windows Vista and 7 have a start-up repair wizard that you can use. For xp, inserting your operating system install disc and running gives you the option to re install your operating system as well as repair the existing one: choose the repair option.
Try this and tell us whether this solves your problem.