1) You say you did a fresh install of WIndows 7 on a new HD. Your PC ran long enough to complete the install? Or did you do that on a different PC?
2) If you boot into safe mode, does your PC remain stable?
If yes, the problem would seem to be more psu or gpu related - the theory being when Windows tries to use the card in graphics mode, either the power supply can't pump out the power, or your gpu fails.
So let's start with that.