Well if im not wrong you need to reset your key before reinstalling Windows else it fails WGA check if im not wrong, but since I havent used windows for quite some time im not sure, but I think a friend reinstalled windows and had to call MS to reser his license key.
That part is correct in a lot of cases, you will have to reactivate your installation key with Microsoft. They allow you to move a retail key*, but not OEM or upgrades which are locked to the hardware and the original license, respectively.
* I think, now, that they only allow two or three moves within a certain time period.
