That's not good at all. You might have gotten a virus of some sort. The next thing I always suggest is doing a Windows XP installation repair using the CD. Just boot from the CD and select the repair option. Hopefully this fixes it.
Oooooh, not good. Something must be seriously screwed up. What were you doing when you started having problems, and how long has this been going on? What Anti-Virus do you use? Have you run HijackThis? If so, sumbit your log here.