You can do this in a few seconds in Win7 with keyboard shortcuts. I don't know if this was possible in XP, never bothered much with shortcuts in it.
Open two instances of Windows Explorer. Press Win+ArrowLeft to put the first Explorer on the left (it'll automatically take up half the screen), switch to the other Explorer and press Win+ArrowRight to put it on the right side.
You can do it the old-fashioned way, too, of course, but the shortcuts are neat once you learn them.
Lifehacker teaches some Win7 shortcuts.