Setting up a backup system with one windows vista laptop and a mac laptop

I've got a household with a windows vista laptop and a mbp running snow lepoard.

What is the easiest backup system to setup for a setup like this? Backing up the windows laptop is more important than the mbp, but I'd like to be able to do it off a single harddrive if possible.

Is it just easier to get two harddrives and manually backup them on a schedule?