It depends on if they bought the system new and from an OEM dealer.
Basically, if you have an OEM copy of XP, the license rights are non-transferable and for things to be legal, you have to buy your own copy of XP. If the person bought a retail copy of XP, then you're probably fine.
If you're simply talking about changing the name that shows up in the About box, there are a number of programs that can change that quite easily. Or you can just open Regedit and scan for that name, then change it to your own whenever you find it.
Is it possible to register windows XP in your name if you've bought a used computer from someone else and it is registered in the original owners name?

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