Because I tried everything else. And before I started any of this, I removed the AMD paste from the heatsink and replaced it with Arctic Silver 5.. Problem was, I did this on two other heatsinks on the mobo and ran out with a little less than I'd like on the CPU.
Luckily I like to keep that stuff around and ordered another 12g as soon as I ran out. I just hadn't gotten around to putting more on the CPU yet. (I knew it needed it)
So I would say if you're installing W7, setup successfully completes everything and on first boot you get some type of error.. I got two separate ones... First, when installing from a internal DVD drive, I got an error and setup would sometimes prompt for drivers. Second, when installing from a USB DVD drive, I got the registry missing or corrupt error... Anyways, I would say check EVERYTHING.
I tried 2 different HDDs. Nothing.
I tried 2 different brands of RAM. Nothing.
I tried 2 different DVD drives, external and internal. Nothing.
I tried removing all PCI(e) devices. Nothing.
If none of those work, check your mobo/CPU. Seriously, as soon as I applied more AS5, it was a whole new computer.