The installation requirements for installing Windows XP, using Apple's Boot Camp, state you can use Windows XP Home Edition or Windows XP Professional, providing they include Service Pack 2. If you installed a non-genuine copy of Windows XP Professional, then you'll need to purchase a genuine "Full Version" of Windows XP Professional and then change the non-genuine Product Key to the new genuine Product Key.
The Genuine Advantage Product Key Update Tool is only valid for users attempting to change their current non-genuine Product Key to a genuine COA sticker or genuine Product Key - all without a reinstall! http://go.microsoft.com/fwlink/?LinkId=50346&clcid=0x409
Boot Camp Beta: Requirements, installation, and frequently asked questions http://docs.info.apple.com/article.html?artnum=303572
