There are plenty of legal copies of XP Pro available on e-bay in the $40 range. For example, this listing has "more than 10" available:
http://cgi.ebay.com/NEW-Microsoft-WINDOWS-XP-PRO-SP3-with-COA-/130410568438?cmd=ViewItem&pt=LH_DefaultDomain_0&hash=item1e5d135af6