Here's a tiny reboot.com.
I believe this is the smallest useful DOS program.
Commands to create it:
debug
n reboot.com
a 100
int 19
< ENTER >
rcx
2
w
q
This creates a two byte com file which will reboot the system, bypassing the BIOS POST and jumping directly to the boot loader. It's very useful when you dual boot between Win9x and another OS. Just create an MSDOS mode shortcut to this program and you can rapidly switch to the alternate OS. However, If you use EMM386, It sometimes fails to reboot.
QuarterDeck's QEMM stealth memory manager had a similar feature called QuickBoot.