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.