VMware Fusion and parallels are two great options, if you have a copy of xp and want to run it on top of the os x interface.
I must say I absolutely comply with the VMware fusion idea, I've been running XP on top of my OS x interface for a while now and even till this day it amazes me how well it works. You can choose to have it running virtually in a seperate window but you can also choose to unite your virtual OS with your OS x running making every piece of software you run from your XP-machine pop up in your mac as if it was installed on the mac itself.
You should definatly try out VMware fusion, it's not that pricy to buy and it can be tested for 30 days!