Hello pintrees,
Carey is correct, but from a Licensing point of view, there are some other complications.
First off, some background information: Microsoft sees the Motherboard as the Key Component of a computer (basically, it IS the computer). So, if a motherboard is replace, a 'New' computer is created. (the Motherboard is the only component that does it)
If your copy of XP is a Retail version, then you have no licensing issue. You can use it on any number of computers as you wish, So Long As it's Only Installed On One Computer At a Time.
If the copy of XP is OEM (either came pre-installed on the computer or was bought as an OEM System Builder bundle) it can only Licensed to be used on the first computer it was installed on. (It would no longer be licensed on that computer if the computer's motherboard was replaced)
Hope that answers your questions,
Darin MS