dual size 10 vista xp army boots

OK... excuse the title... but here's my situation. I'm running XP on a 40 Gig drive and have a free 20 Gig drive on which to run Vista. Since active brain cells are at a premium this weekend, I am looking for the simplest path to do a dual boot with both OS. I am more than a bit apprehensive in doing this and like the notion of having an XP safety net available.
Suggestions folks? A free case of LaBatts or Molson to the first person to come up with an approach that will get me through this. ;-}