There is no DOS in Windows XP , and you can't boot to DOS, short
of booting from a DOS diskette. And if you do boot to a DOS
diskette and your drive is NTFS, you won't be able to see it
unless you use special software like NTFSDOS.
But what is it you want to do? If you want to reformat and
reinstall Windows XP, just boot from the Windows XP CD and follow
the prompts for a clean installation. It will do the reformat for
you.