This is the way WPA works in Windows XP: http://www.aumha.org/win5/a/wpa.php
What does ‘substantially the same’ mean? WPA asks for ‘votes’ from each of these ten categories: ‘Is the same device still around, or has there never been one?’ Seven Yes votes means all is well — and a NIC, present originally and not changed, counts for three yes votes! Minor cards, like sound cards, don’t come into the mix at all. If you keep the motherboard, with the same amount of RAM and processor, and an always present cheap NIC (available for $10 or less), you can change everything else as much as you like.
In other words, you can change three components besides the NIC (since everything else gets only one vote) without freaking out WPA. After 120 days with no further changes, this becomes the new "normal" for the system and you can make further changes without reactivating.
But like just brew it!
, I don't know if Vista/7 change this. I suspect not.