Find the shortcut to calculator in the Start menu, right click and select "Properties". You can enter any shortcut you want in the "Shortcut Key" field.
"Not a true shortcut key combination, but a comfortable way to do it anyway. win-key+r [for the run window], type calc and press enter.
Might seems like a bit of work relatively for a shortcut, but if you're not completely rubbish at typing, it's easy and quick (and if you use the method for other programs you really get used to it)."
