Don't want to throw Luckystar under the bus but, This "How To" can not be easier. I know... that's how I did it!
This is the way to do it if you have XP already installed. How to dual-boot Vista with XP (with XP installed first) - step-by-step guide with screenshots Now, this is the way to do it if you Vista is already installed. How to dual boot Vista and XP (with Vista installed first) -- the step-by-step guide with screenshots