Might not be exactly what you want, but some front end packages like ArcadeFX have options to drop to OS, Shut Down, or Reboot. You could select it via joystick and player 1's button1.
If this isn't what you want, it wouldn't be too much trouble to code something or to set up a key'd macro to start the shutdown process.
you could also create a shortcut with the following command:
for XP: SHUTDOWN -s -t 01
for Win9X: C:\windows\rundll.exe user.exe,exitwindows
Hope this helps,