|Probably a virus.|
1st step, run a virus check from 1 or all of these. You may even have to Slave it to another hard drive / comp ( no need to bolt it in, just cable it & leave it outside the case ) & copy your stuff onto the other HD.
You may have a choice of jumper settings on the master HD, such as master with slave or MS.
Online Virus check ( free )
Run the following free, online virus scans.
System Restore option in Windows Me/XP
Users of Windows Me and Windows XP should temporarily turn off System Restore. Windows Me/XP uses this feature, which is enabled by default, to restore the files on your computer in case they become damaged. If a virus, worm, or Trojan infects a computer, System Restore may back up the virus, worm, or Trojan on the computer.
Windows prevents outside programs, including antivirus programs, from modifying System Restore. Therefore, antivirus programs or tools cannot remove threats in the System Restore folder. As a result, System Restore has the potential of restoring an infected file onto your computer, even after you have cleaned the infected files from all the other locations.
Also, a virus scan may detect a threat in the System Restore folder even though you have removed the threat.
For instructions on how to turn off System Restore, read your Windows documentation, or one of the following articles:
How to turn off or turn on Windows XP System Restore
Download HiJackThis, install & run to get a log file. Don't fix anything.
You then post the log file at the site provided below & it will tell you what to fix.
Here are ways to read the hijackthis logfile.
HijackThis log file analysis & repair ( online )