My F12 function key has been hijacked by another program. I know this because when I start HyperSnap (a screen capture program) it first tells me that it can't register the F12 key as a hot key (for screen captures) because it may be in use by another program. Since nothing else is running (I did so as a test) and I have no idea what this other program might be, I was hoping there was a registry entry that maps the Functions keys globally but cannot find an entry in the registry. The closest idea was a suggestion to check "HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Control\Keyboard Layout" but there is nothing in there to suggest any mapping of the Function keys.
The best answer I can give you is to use the SysInternals Process Monitor and track what gets called. I would do the investigation this way.
This should work because you know that you selected and typed something into notepad. This marks the moment where you should be looking at since you will get a lot of information back (your system does a lot of things even when its 'not busy'). It's not easy, but that's really the way to find out what is running on your system. Once you find what the process is that calls F12, you can use the Process Explorer utility to try to kill it (also from SysInternals).