Wouldn't hurt to check out your hard drive to see if it's healthy. Check your free space and then, if it looks Ok, run a chkdsk on your C drive. Open up Properties on your C drive, select the Tools tab, and click Check Now. Click the two checkboxes for "Automatically fix errors" and "Scan for bad sectors". When you click OK, you'll probably get a message saying chkdsk can't gain exclusive access to the drive, do you want to schedule it for the next reboot. Click Yes and reboot. After it comes back up, defrag the drive. Finally, run a diagnostics program on your drive. You should be able to download a program either from the manufacturer of your computer or the manufacturer of your hard drive.
BTW, I assume you have scanned for viruses & trojans.