The question you posed about rebooting your XP Home into MS-DOS mode
is summed up by simply saying their isn't an option for that. The
reasoning I do believe for this is that most people using XP are also
using the NTFS file system which dos doesn't recognize. So basically
even if you booted into dos you would not be able to utilize the
drives which are using NTFS.

This might help you though. It's hard to tell because you didn't go
into why you were trying to boot into dos mode. But if you click on
your start menu, go to run.. in the dialog type cmd.exe and press
enter that will bring up a command dialog similar to that of a dos
command prompt.

If you were looking to access dos for the reason of partitioning,
formatting or other things you would typically do from a dos boot
disk... this can still be done. If this is the case, plz let me know
and I'll further instruct you on the steps.