This works in w95 for sure not sure of later versions
start/run/sysedit
click system.ini
change the line
shell = explorer.exe
shell =clock.exe

Now when you reboot the 95 machine the clock is your shell -close the clock close Windows. After first reboot the only way to undo the above is restart windows in ms-dos mode and edit system.ini w/edit.

Above from training provided to me when I worked the hotline for Compaq.