Windows 7 serial number reload before boot up

i was searching & downloading serial numbers for xp on my desktop, i thought it popped up with a download-able save box, when it was actually an install box on my windows 7 desktop, and i turned and hit enter.
needless to say i now boot to a black screen and am trying to figure out how to reinstall the correct serial number into the registry without a total reinstall as everything i have is on that drive.