Must have been the F5 "Refresh Password" key... ;)

I use Microsoft Keyboard Layout Creator, it's free and allows you to configure pretty much any key or key sequence, for instance to make the AltGr+c combination return the "¢" cent character or any sequence of up to 4 characters, like "bu/a" for AltGr+b, but it won't work for you, as it does not allow to reprogram the function keys.