Have you run any full virus scans (updated AV program) since this happened? I would also run SUPERAntiSpyware or something similar.
You might also check Event Viewer for possible errors
noted at approximate same time.
How To Use Event Viewer - http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
Lastly...you might run the chkdsk /r command. Start/Run...type chkdsk /r and hit Enter. Type Y in response to onscreen query and hit Enter. Reboot the system, the command will execute. Upon completion, the system will boot into XP.