This guide will get you there.
http://apcmag.com/how_to_dual_boot_v...step_guide.htm
If by clean boot you plan to format, it may be better to install xp first, then when you install vista it's bootloader finds xp and gives you the option. In the above link there is also a tutorial on how to dual boot with xp installed first, might want to read both and see what suits you best.