Geek:
My guess:
Virus, or corruption of the Registry, or hard drive is going bad and corrupting operating system files.
To rule out the last guess as the cause, try this if you have Windows 2000 or XP:
1. Click Start > Run
2. In the Run dialog box, type CMD and press the Enter key.
3. In the command window, type
CHKDSK C: /F /V /R
and press the Enter key.
If the result displayed is "windows has ... found no problems" or zero bad sectors, then your hard drive is OK. If the result shows a number of bad sectors found, then your hard drive is about to die, and you should backup as many files as you can to CDs or floppy disks before it crashes.
Do you hear any unusual sounds coming from inside your computer case, like grinding noises or a metalic pings? If so, this is another sign that the hard drive may be going bad.
Let us know how it turns out.
Best regards,
Doc