Originally posted by UP
If you mean it's not working as in passing activation, all you have to do is call the microsoft support line, they'll ask you a few questions and they'll give you an activation number
Indeed! I've done it several times, call microsoft and tell then that you have reinstalled windows and that you lost your key and just as 1_UP said, they will give you an activation code.
I think you can also do it over phone without having to talk to a agent but you have a limit of 5 times I think...