Usually not. Microsoft takes a snapshot of your hardware when you activate it, if you try and reactivate it after install and it's different, it may not go through. Usually it's 1 device per key, not 1 user per key. I know.. it kinda sux.
It depends on whether you have a Retail version of XP Pro or an OEM version. According to the Microsoft EULA, Retail versions can be transferred from one computer to another but the OEM version is tied to the motherboard of the first computer it was activated on.
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