Tell me what you want to do. Do you want to dual boot or just have XP? I would say dual boot.
http://apcmag.com/how_to_dual_boot_vista_and_xp_with_vista_installed_first__the_stepbystep_guide.htm
This is a pretty good tutorial. There are several parts and they walk you through it well. If you have any problems or questions let us know. If this doesn't help post up what type of PC you have, if the hard drive is partitioned and what you want to get done and we will help ya out. Its really easy but can appear intimidating.