I recently suffered a dead CPU, system would turn on, fans would spin but no POST or beeps.
I'd see if you could get hold of another one to try. Given that it was your cooler that was changed shortly before it died, I can't see why CPU wouldn't have been considered one of the main culprits.
Sounds like I've got the same issue as you.
Honestly fellas, I've tried everything, including all the things you just mentioned. My rig is currently sitting on a cardboard box. It can't be a short. I reseated the CPU again last thing before I went to bed last night. Nothing is visibly wrong with the underside of the CPU and there are no bent pins in the socket.
The current theory I've arrived at is this: my CPU is almost 4 years old. I never did overclock it but I was using the stock intel cooler and the temperatures on it were abysmal. For most of its life it idled at over 60C! Must have been a bad seating or too much thermal paste, or too little, plus I only vary rarely cleaned out my case so it was very dusty most of the time, which didn't help. So I think that that heat wore out the CPU and when I put my new GPU in and put my CPU under heavy load by playing Battlefield 3, it just died...
How realistic does that theory sound?
Also, I'm still on the lookout for a C2Q. If you've got one you don't want let me know and I will take it off your hands...
