I didn't have any issue with normal windows usage either. And I did have a gpu fan as well but it seemed to be failing.
But I didn't know my GPU was overhetaing at first because windows itself was stable. I only got crashes when I solicited my GPU with games which seemed to cause my gpu temperature to rise.
However a test I did pointed me toward the GPU overheating direction which you might want to try.
I got an overclocking utility of some kind for ATI boards that had a stability test (forgot which one) and I severely
underclocked my GPU core clock speed and memory speed (by over a 100 mhz each). Since they were underclocked they generated less heat and were more stable. Test this for like half an hour. If it doesn't reboot or crash, taht's good.
After that, test the same utility but with your normal settings (default clock and memory speeds) and see if your system reboots or hangs, and especially how long it takes for it to crash. Also check out if the utility creates artifacts because that's a sign of overheating.
When my system was undercloked, my system was rock solid but at stock speed I crashed in less than 5 minutes and produced artifacts like crazy. Drivers didn't change a thing.
I found out my memory chips were overheating using that test. To fix it I simply used one of the case fans already in my box (I have two) and stuck it near the card. It still cools the overall system but since it blows fresher air on the card, it doesn't overheat. I ahven,t had a single crash since and it's been over 6 months. I definately reccomend trying that.
It's all much simpler than it sounds.
edit : I did a little search. I used ATI tools to test my card. You can find it there
http://www.techpowerup.com/atitool/When you start seeing artifacts every frame, it means your card is overheating. The utility even has an auto overclock feature. Since my card was overheating on its own, it severely underclocked my card on its own! This was the greatest tool in trying to fix my issue.
I'm also pretty sure that if you try other graphic intensive games, your system will hang. Your problem is just exactly like mine and I was convinced it was a game compatibi.lity or driver issue myself until I tested it further.