You may have a failing hard drive, If you have data that you cannot afford to lose, remove the drive from that PC and install it as a secondary drive in another PC, then copy any data you want.

Do this first, any other tinkering with the hard drive could make any data irrecoverable.

 

Once your data is backed up, try this  http://kb.wisc.edu/helpdesk/page.php?id=5097

If the dive is mounted in another PC, you can run check disk from that PC, ignoring the link above, you can do it this way http://www.updatexp.com/windows-xp-chkdsk.html   read this section " Windows XP Chkdsk - How To Using The Graphical Version..." 

Except you will right click on the problem drive that you installed as secondary drive in that PC.

 

Remember back your data up First or risk losing it.