Without knowing anything else, I would run chkdsk on the system, using the Graphical Version procedures as detailed at XP Chkdsk, Using the Graphical Version (My Computer) - http://www.updatexp.com/windows-xp-chkdsk.html
When options are presented, be sure to put checks in both the "scan" and "recover" boxes.
Reboot the system, let chkdsk run to completion. It should reboot (IIRC) upon completion.
Chkdsk Explained, Understanding What Chkdsk Does - http://support.microsoft.com/kb/314835/EN-US/