Insert a floppy disk containing DOS, such as win95, 98, or ME. Reboot the
PC.
Note that XP will make an ME-type boot floppy for you.
However, if the purpose of getting to "DOS" is to fix something, be aware
that XP usually comes with the NTFS file system, and DOS can not see that,
without special drivers. The read-only drivers are free, but the read/write
drivers are pricey.
An alternative to DOS is a bootable CD or DVD with LINUX, such as KNOPPIX,
which is free. The latest version can boot most PCs and gives full access
to all windows hard drive partitions. KNOPPIX does NOT install on the hard
drive, so it is "safe" to play with. By default the access is read-only,
for your protection. But, you can change that with a mouse click.
links:
http://www.knopper.net/knoppix/index-en.html
http://www.knoppix.net/wiki/Main_Page