Ok, so I was changing the cooler on my hd4850 to the accelero s1. After fitting it I put it in my system and wouldnt boot properly. Itd restart when the windows loading screen comes up. So I figure that the card must be overheating or something and refit. After doing it correctly and refitting the old HSF, theres no difference. I must have borked my card, right? So I use my backup card (2600XT) which is known to be working and I have the exact same problem. Restart at the windows loading screen. Memory passes memtest. Not a lack of power from my PSU either. It posts okay, cpu is at stock. Only keyboard and mouse is connected. So whats wrong? Thanks.