I actually prefer Vista, as when I had XP, I got so many fatal crashes and blue screens, Vista I never got one. Most of the errors correct themselves as well, not to mention higher security, so less viruses.
As for your XP, there IS an XP version of this, but I've never tried it, so use at own risk, although it's pretty safe to do.
This is a guide for turning XP into the OS X (I'll find a guide to doing the Leopard for XP later): http://features.engadget.com/2004/06/09/tu...-pc-into-a-mac/