Thats typical. Vid card / cpu type, etc are all logged for machine when XP is installed. CHange too much hardware, and yes, re-activation needs to be done. You should still be able to do it via the inet without calling them. Otherwise a call to MS for a rediculously long activation code is in order.
I have personally experimented with this activation though and it seems you can activate it, and then ghost the image and if you have the exact same hardware, you can have it working on 100 different machines if you want without it realizing that it needs to be reactivated.
Just some results from my experiments.
-S
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