There is no true DOS in XP. The closest thing is the command prompt, whihc
supports most DOS commands, like COPY, ERASE, XCOPY, etc. If you need to
get to a more fundamental level to fix something, first try SAFE mode. If
that is not enough, then try the XP recovery console, run directly from the
XP CDROM. However, be aware that the recovery console is much more limited
that good old DOS.
A win98 or ME boot floppy will still boot a PC into true DOS. XP will even
make you an ME floppy as a format option. But, DOS (real DOS) will not see
any NTFS formatted disk partitions, without third-party drivers.
Finally, it is possible to make a dual boot with XP and 98 or ME. But, that
can get complicated, so please read up on how to do it before you try it.