Do you have a Microsoft XP install CD?
Have you checked Event Viewer for possible clues as to what might the problem or problems?
How To Use Event Viewer - http://www.bleepingcomputer.com/forums/t/40108/how-to-use-event-viewer/
Pay attention to errors only, no informational items.
System manufacturer and model?
Have you tried running chkdsk?
Running CHKDSK - http://www.1securitycenter.com/windows_xp_faq/chkdsk.html
. Follow the 8 steps outlined, with the exception that you put check marks in both options (Automatically Fix, Scan/Recover).