Do you know if your PC is clean or not?
Was there anything that happened prior to this issue that may have caused the problem?
Sometimes bad memory or a bad hard drive may cause the problem you have now. I would first try to see if the memory went bad. What you can do is open up your computer and swamp out the memory with known good memory or if there are multiple sticks in there, try one at a time. I would also recommend you perform a memory test. You should definitely get UBCD
and test your memory and hard drive.
If this is not a hardware failure and a virus, load a live CD with anti-virus scanner or plug your hard drive (or use a USB HDD enclosure) in a clean computer with anti-virus and scan it from that computer when it's booted. Another possibility is that Windows is corrupt. You would have to try to reinstall Windows over your current installation, attempt a repair, or wipe it on and make a new Windows installation.