If you look on your product info that came with your XP cd, most say they can be used to 2 home computers.
I know that XP Pro is registered upon installation (it pops up asking to register, and if you don't you get an annoying pop up from time to time). If you have XP Home, I don't know if it registers or not.
But in answer to your question, I believe you can have XP installed on up to 2 separate computers under the same CD key. (check to make sure before you do though).