I'm assuming that your run a good virus checker as this is also a classic sign of an infection:
Symptoms of Viruses
If you encounter corrupted files, unreadable disks, a significant decrease in available disk space, programs that won't launch, unexplainable out-of-memory errors, or other erratic behavior, scan for viruses as part of a more extensive troubleshooting procedure.
Try several differnet virus checkers including free online ones!
Cheers