The major drawback of going with an OEM version of Windows.
It is the cheapest way to go, but one of the reasons it is so cheap is BECAUSE YOU CAN ONLY INSTALL IT ON 1 COMPUTER. New motherboard and processor, you are most likely going to have to buy Windows again...unless you can come up with another working key.....which can be done, but it is not legal, so we won't go any further with the discussion.
Or like tecmo34 said above, if you can convince Microsoft that the board was defective, they may supply you with a new key. Sooooo, this may limit you to replacing your current board with something very similiar, except more up to date maybe with more features. But if you change to a completely different board, you may indeed have a problem that you cannot fix without digging into your wallet. Good luck!