Lawl it's odd that it only gave you one use( because usually it works an infinite amount of times on ONE system, but when you throw it onto others then you run into a couple problems), buuut you can call microsoft there should be a number there on your screen when it comes up asking for your key. You'll usually get an automated thing. You'll get a key after a few questions just tell them you replaced your motherboard or something ._. sigh... Thats microsofts way of trying to bone piracy, but they're failing.
They want people to buy separate keys for each of their computers AFTER the fact that they spent about $300 on their new operating system, cause Gates is all about the money.