If data is deleting it could be a virus, always best to run a scan of your
system with up to date antivirus software because, unless we are aware of
something specifically going around with specific symptoms, your antivirus
software is in a much better position to answer that question, we'd only be
guessing.

Another possibility is that the hard drive is failing.  If your hard drive
didn't come with a tools disk, go to the drive manufacturer's website,
download their tools, make not of their instructions for creating the disk
and running their tools.  Then run their diagnostic.  XP's tools are not
robust enough to diagnose this kind of an issue.

--
Michael Solomon MS-MVP
Windows Shell/User
Backup is a PC User's Best Friend
DTS-L.Org: http://www.dts-l.org/