If you really want to boot into DOS then you will need to install DOS on a separate partition and boot from there. XP isn't built on DOS like its predecessors. In XP DOS is an add-on program.
Another option is to boot into Safe Mode with Command Prompt. Then you could make use of these command line [microsoft.com] codes.
A third option would be to look for an old DOS boot floppy from a previous version and try that out. There are several sites on the net that have preconfigured boot floppy setups that you could download and try out.