Reinstall OS X but when you do, use Disk Utility to create a very small partition for it (2-3GB would probably do it). Then when you install OS X, customize the install to reduce the number of packages to be installed (hence minimalist install - but make sure that once you've installed it you delete all the extra crap that might have remained such as iTunes blah blah blah just incase). Once you've got the system up and running, immediately install XP with boot camp using the remaining HDD space. Once its all up and ready to go, reboot into OS X and in the System Preferences, select the XP partition as the boot partition. That way the computer will default boot into XP, you can press option to go into OS X if you want, and you'll aready have a fully operating OS including NTFS read support that can help recover any problems you might have down the track with XP. (Make that read WILL have with XP, the whole idea of what your trying to achieve just runs completely contrary to what any sane Mac purchaser should do IMHO
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