Literally you have lost it. There is no DOS in Windows XP (that's
largely what accounts for its much-improved stability), and no
way to get there, other than booting to a DOS diskette (and if
your drive is NTFS, booting from DOS won't see it, without
running special software).
But there is a command prompt (go to Start | Run and type CMD)
which emulates DOS and will let you do almost everything you
might want to do there.