The little known tidbit is that activation data isn't stored on MS's activation servers indefinitely. I don't know what the time for Win 7 is, but it used to be 120 days for XP Pro, and I have successfully activated Vista Ultimate with the same product key on another machine after a period of six months.
Also, you don't need the full version to install Win 7 Pro. The upgrade version will work just fine, but takes a bit of extra time to install. There's a loophole that was first discovered in Vista, that Microsoft allowed to continue in Win 7, so it appears that they condone it, otherwise they would have killed it in Win 7.