Go with XP. I've been working a lot with Vista and I hate it. I never like new systems initially but then usually come around, but with Vista it is taking a lot longer than ever before. It's hardly more functional, and they changed the labels of things to not make as much sense as it did before.
For example:
Add or Remove Programs is now -- Programs and Features
How much easier could "Add or Remove Programs" have been?
I just got a new computer for home, and I went with XP. Solid.