If the PC is one that was purchased with XP on it, then most likely the license is OEM and cannot be used on another machine. Even reusing it on the same machine that has a new motherboard installed is iffy at best (it depends on the mood of the M$ tech you talk to).
If the license was purchased separate from the machine, then it would depend on if it was OEM or Retail. As long as the previous installation is deactivated, then a Restail license can be reused on another machine (I don't recommend doing that unless it's intended to be temporary since XP will reach its end of support 04/14/2014). If OEM, technically, you can't reuse the license on another machine although many have had luck sweet talking M$ into allowing it. Since XP is "dying" soon anyway, I wouldn't bother. If you haven't bought it already, I would suggest getting a retail version of Win 7, even though it costs more, to ensure you will be able to transfer the license in the future, especially if M$ tightens up their restriction on reusing OEMs (something I expect will happen based on the direction M$ has been heading lately).