I'm not in any position to give legal advice, but I suspect that your EULAs for the 3 legit copies limit your licenses for installation of each OS to only one machine. Besides, if you try to register one of the XP copies for the Gateway machine, you'll probably be denied because of the key already being used by another one. So that means you won't be able to download updates, among other possible problems (perhaps also negating your license to the original machine it's installed on).
You should probably verify your warranty under Gateway before you change OS, in any event.
I'd be happy to be contradicted by someone with more specific knowledge.
Edited by Dialer, 04 January 2008 - 04:14 PM.