I have used my retail (upgrade version) of XP on several machines - but only on one at a time of course - and never had a problem with activation although I have had to call Microsoft several times to explain what I have done.
As others have said, retail versions can be moved from PC to PC, as long as it's only on one PC at a time.
If you move it often you may find that activation fails and you will need to telephone MS for manual activation, this is because the hardware is scanned during activation and a checksum based on the serial numbers of various components is passed to the activation servers.
This data disappears off the server after 6 months, so if you wait at least 6 months between hardware changes you experience any problems.
OEM licenses are different, that license is tied to the PC and cannot be reused