You can get a DOS boot disk, actual several flavors of them, from
www.bootdisk.com. These are self-extracting files that will expand
out onto a floppy. Leave the floppy in the drive and reboot. You
will be in true DOS.
I personally like their win98SE bootdisk with generic CDROM support.
Note that pure DOS will not see any NTFS partitions on your hard
drive. However, there are free read/copy-only NTFS drivers for DOS
available at
www.systeminteranls.com.
Also, DOS does not support USB. However, there are some drivers out
on the web that work with some hardware configurations. Try a google
seach if you need that functionality.