Quote:
...legit, previously registerd copy of XP on a different machine...


As long as it's a retail copy, rather than an OEM copy, then there's no problem with this, as long as you're no longer using it on the old machine. Where you do run into problems is that OEM copies are licensed specifically for the machine that you purchased it with. Which means, for example, that I've got a copy of XP Home that I can't use (because I replaced it with retail XP Pro on that particular machine).

As the others have said, you might need to phone them up if on-line activation fails.

I've done it a couple of times, and I've never had to phone.
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-- roger