Contact MS directly with regards to their terms and legal issues.
BV Edit: But the real answer is something to the effect of "once activated on installed hardware, it cannot be activated again", meaning one motherboard, one HD, one everything, period. "Replacement" motherboards with built on network cards is a gray area due to NIC's having different MAC addresses and that is still (as far as I am aware) part of the hardware hash. Again, contact MS for their specific terms.