If you totally formatted your drive, then a virus is pretty unliekly, if not it could be the problem. If you are good on updated drivers and such, you have a hardware probelm. Most likely memory, processor or motherboard. If you can swap out the ram chips one at a time. Reseat the CPU and all cards in the system, make sure all the fans are working. You might also try a different video card if you have one. Finally some PC makers, Dell in particular, have a system test CD you can download and boot from, then test the system. That might also find your probelm.