when you install XP part of the process asks you where and how you want to install...choose to reformat using NTFS, then load the OS...of course this method deletes access to all data on the HDD (though data recovery tools would recover a fair amount)

 

Or, you could just install the Win XP Pro on the FAT32 partition. XP works happily with FAT32, although there's a few 'File Security' options that won't be available, but they're more for 'Commercial' file security rather than home files.