I upgraded a client's system today (motherboard, ram, cpu, and vid card). Fully expecting to have to repair or completley re-install, to my surprize, the thing went right into windows, detected almost all of the new hardware, and asked to be rebooted. On the reboot, windows popped up a warning that said the system had undergone major changes, and that it had re-activated within 3 days. Few clicks of the mouse it was re-activated, without having to call MS. Sometimes windows does surprize me, pleasantly. So yes it's possible, but I think the system today was a rare instance, and I wouldn't count on it going as smoothly.