Too bad Microsoft can't really track if you've transferred it to another computer or not. I've transferred my OEM copy of XP through 3 computers already. Each year they allow you to activate your copy of Windows again, in case nobody knows that...Or so it seems.
If you try to activate it again in the same year (if you have to reinstall Windows for any reason), of course you'll get a message saying you've exceeded the amount of times activated on that serial. You just call them and tell them that you only have it installed on one machine and then they'll give you an activation key. So either way, you can activate it by waiting one year, or calling them on the phone.
I think Vista is harder to get by this time around...