I agree with pathena, you have to buy the installation software of the windows XP in order to have it installed in your mac. Another option is if you have a friend who have the software you ask him/her if you can barrow it and have it installed in your pc, provided that in the registration process it will be registered in the name of the owner.
Yes I had xp done on my Mac a while back at the bookstore at the college I was attending they didn't recommend windows 7 on it? But anyways i remember it cost around 120$ some student did it for me after buying the program. I really wish I had the windows 7 on there tho : (