I just saw this one on Bink. It might be handy for anyone who’s evaluating Vista or like me, is not too keen to use their limited number of TechNet/MSDN activations in lab work but find the default 30 day window to be too restrictive. There’s a command you can run (slmgr -rearm and then reboot) that extends the activation grace period. You can do this three more times to extend the grace period up to 120 days.
Credit: Bink.