I Personally would highly recommend against installing Windows XP Professional x64. Not just because it was crazy buggy and had more application compatibility issues than Vista Beta did, but also because you have only 1.5GB of RAM. There is literally no advantage to installing Win XP Pro x64 on that system.
But to your question, as Espi said - You would either have to purchase a new license or "Obtain" a new copy. THEfog -- 9xx SOLDIERS SANS FRONTIERS
