Holy cow, I wrote a book!
You can set a hotkey on the shortcuts in your Start menu,
and Explorer will launch that shortcut when you press the hotkey,
but not if you are in a fullscreen application,
like when you set Paint into fullscreen mode,
or when you are displaying a PowerPoint presentation.
Why are shortcut hotkeys ignored when a fullscreen
application is running?
This feature was added by customer request.
The issue is that you're playing a game like
World of Warcraft or
and you're mashing your keys like crazy,
and whoops you slip and hit
Ctrl+Alt+C instead of
and now you inadvertently launched Calc or something.
Whack, you get taken out of your game into Calc,
and by the time you get back into the game,
your character has been mortally wounded,
and you've lost the game.
To prevent this from happening, Explorer temporarily
ignores custom hotkeys for Start menu shortcuts
when a fullscreen application is running.
Note, however, that there is a bug in older versions of Windows:
If you press a hotkey for a Start menu shortcut
when there is a fullscreen application
running, the hotkey is blocked.
(So far so good.)
But even if you clear the fullscreen application, the hotkey
is still blocked because the hotkey got "stuck"
in the blocked state.
To unstick the hotkey, press a system hotkey involving the Windows key,
then try the original hotkey again.
(I use Win+B
since it is relatively harmless.)