On a Mac? Need Windows XP otherwise? Need a clean OS to test stuff on? Time to virtualize, and do it for free. VirtualBox + XP Performance Edition for the win. Legit? Meh. Works great? You betcha.
http://www.virtualbox.org
http://thepiratebay.org/torrent/4577405/XP.Pro.Performance.Edition.December.2008-TJ