Windows XP and the other windows which use the NT structure do not boot into DOS and then load up windows like 95 and 98 used to. But the command propmt (cmd.exe) in XP is fully fuctional. These days i cant think of any reason you`d want to install DOS on a seperate partion. But if you have some need for it i`ll give you the links to FreeDOS and explain how to install it along side XP if your not sure on how to partion your drive.
Mark