According to the EULA for retail Windows Vista, you will only be able to transfer the licence to another machine once.
Yes you read right, once, for a retail copy, not OEM.
This royally shafts those who choose to build their own machines. Upgrading the motherboard & a few other things meant that re-activation under XP was a pain, but at least it could be done.
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The more I hear about Vista, the more I like XP.