Does your XP machine have a License sticker?
In theory, if you have a retail copy or a purchased computer. As long as you remove XP from the current machine and put it on another, you shouldnt be violating your license but it may ask you to validate your license. Without removing the XP install from the desktop, this would be illegal.
Either way, since XP will no longer be available without corporate workarounds in June. I would recommend you buck up and get used to Vista because its going to be the standard for the next couple of years. Even businesses are going to be limited on their OS options as the months go by.