Windows XP is using NTFS 5.1, a minor upgrade from  NTFS 5.0 that shipped with Windows 2000.  The easiest way to switch between NTFS and FAT32 is to use Partition Magic.  It will do the deed without losing data.  Keep in mind your hard drive has to be "basic", as set in Computer Management.  As well make sure you're not using disk ratios or encryption.