I've had this exact effect in several games before (including S1), It was completely driver related every time. The artifacts looked identical to the picture shown. No overclocking, no dodgy hardware.
Always try a different driver first, it takes 5 minutes to do. Also the newest driver is not always the best one to use for your card.
If you can't resolve the issue by trying different drivers, then look for more serious issues. Until you eliminate the obvious and basic things you can't really be sure.