you probably don't want to replace install when your going to dual boot, your going to need a dual boot software (you don't need but i recommend) and try to get one that helps you install, with vista that's my only choice i hope its not the same with XP.
if you have already installed windows try the anaconda partition manager in fedora 9 it is great on dual boot and gives you option screen as well as allowing default o/s option,so Fedora is my dual boot choice and works sweet,a monkey or my 11 yr old can install dual boot so it isnt too hard.
Ubuntu uses grub and fedora uses anaconda partition managers.