use boot camp to install xp and setup everything, it's pretty straight forward just look on apple's site. once you get xp installed via boot camp, you can use parallels to boot from that exact same partition. this is what i do and it is very convenient. if you need to go into windows to do something really quick in say matlab or something, you can just use parallels and there is no need to restart. however, if you want to go play some cs: source or something, just restart and boot into that exact same partition via boot camp and play. it is the best of both worlds using 1 single partition. the end.