Just note that if you buy a Vista
upgrade as opposed to a full-blown copy of Vista, that it needs XP in order to perform the upgrade. And as part of the Vista's EULA you forfeit your rights to keep running XP as soon as you upgrade, so to leave XP running on a separate partition of your hard drive would be illegal as it violates their EULA. (Plus, you'll never be able to reinstall that copy of XP if you ever need to wipe the hard drive and start anew.)
http://digg.com/software/Vista_upgra...es_your_XP_key
The only legal way to dual boot XP and Vista is to purchase the full-blown copy of Vista and install that on a separate hard drive partition, unless you feel like
installing Vista twice to work around this annoyance.