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You say your MBP came "preinstalled with Windows XP." If it's "preinstalled" why are you trying to install it again?
Try this: tell OS X to reboot. When you hear the chime, hold down the Option key. After a bit, you should see two disk drive icons, one for OS X and one for XP. If you do, that means that everything has been properly installed and all you need to do is select XP and the machine will boot into that.
Please tell us how this plays out.