Well you can. Just insert your XP disc and go thru the setup. But when it comes to where to isntall jsut dont use the default Windows folder that it wants. Create a folder called XP or something like that.
A bit of advice. Since you did not partition i would suggest you get PArtition magic or GParted and create a partition with your free space. Then isntall XP. Having 2 OS's on the same drive at any time is nothing but trouble waiting to happen.