There's no DOS in XP. If you can find a DOS boot disk (try
www.bootdisk.com), and if your computer has a floppy disk drive - or
if you make a bootable DOS CD, I suppose - then you can boot into DOS.
If you are using the NTFS file system, you won't be able to see your
hard drives.
Technically, WinXP _is_ a DOS (Disk Operating System) - of course, so is
Linux, OS/X, Solaris... People like to say there is no DOS in XP
because they feel that an association with the old MS-DOS is a bad thing.
Current versions of Windows have their own kernel and are no longer just
a shell running "on top of" a simpler non-GUI system like Win95/98 were,
but the older MS-DOS influence is still strongly in there (why else is
there an unused B: drive letter on our machines?).
[End rant mode]
To partially answer your question:
You can boot from a DOS floppy or CD (try
www.freedos.org), or format a
DOS floppy in WindowsXP and make it bootable by selecting "Create an MS-
DOS startup disk" - puts on the WindowsME version of DOS.
Some DOS or Linux boot floppies/CDs have some (often limited) NTFS file
access support.
You can also use NTFSDOS from
www.sysinternals.com under DOS - the free
version allows for file reading, paid version needed to write.
Some versions of Symantec Ghost come with omnifs, which allows access to
NTFS as well as other filesystems from DOS.
Most of these methods of accessing NTFS from DOS are not the same as
native support. Often you do not specify the partition by a drive
letter, and cannot simply use the files as you would in DOS. You can do
some operations such as renaming/copying/deleting files, but to edit file
contents you likely would need to copy the file to a partition that DOS
natively recognizes, edit it, then copy it back.