Error installing XP SP2

I’m trying to install XP Service Pack 2 on a Macbook Pro – at the time I installed windows, the only available license I had includes neither service pack. The partition dedicated to Windows is 15+ gigs, yet when it tries to make the backup for uninstalling SP2, it keeps saying it doesn’t have enough space. Is there any way to disable it from doing so, or a work-around, or maybe something I just overlooked?