if you have a MacBook, which is powered by intel processors and not Powerbook/iBook, you can install windows on it with apple's bootcamp. if you have the pre-intel books, then you would need Virtual PC.
for one thing, virtual pc wont work with intel processor macs because the whole program has to be rewritten all over again. thats why microsoft just announced they are shutting down virtual pc development.
anyways, if you are really using a MacBook, then you need to just grab a Windows CD, and use bootcamp install it. Then you will have both OS X and WinXP but you will have to boot into Windows if you want to use your windows apps.