What operating system?
Is it properly updated for critical updates?
Have you tried running chkdsk in the Graphical Mode...and then defragging after that?
XP Chkdsk, Using the Graphical Version (My Computer) - http://www.updatexp.com/windows-xp-chkdsk.html
Put check marks in both options (scan, recover).
IMO, it appears to be a system problem (maybe file corruption)...lots of references on the Web, but no answers that I can interpret
If it is Windows corruption, running sfc /scannow may correct it.
LEARN how to use SFC.EXE (system file checker) in this article! - http://www.updatexp.com/scannow-sfc.html
You will need the Windows CD to run sfc /scannow.