How to toggle between different monitors with one key on Windows 7?

I'd solve this problem with a combination of Nirsoft's MultiMonitorTool and Chris Mallet's AutoHotkey. The former can accept command-line options to specify which monitor(s) to enable and disable; the latter offers a scripting language in which you can bind arbitrary actions, such as calling an executable with a given set of command-line options, to arbitrary keystrokes. (It's not what I would call a good scripting language, but when the alternative interface to the functionality involves writing Visual Basic script that's even hairier...)

Both are free, and neither is heavyweight; the only overhead involved is in keeping an AutoHotkey script running to catch the keystrokes you've bound to change monitor settings, but that lives in the system tray and idles any time it's not dealing with the specific keystrokes it knows about. Add it to Start Menu\Programs\Startup so it runs when you log in, and that should be all you need.

There isn't a windows built-in for this as far as I know, but if you have multiple monitors, I strongly recommend that you install UltraMon. It has support for (among many other things) creating custom profiles for display configurations (enable/disable monitors, setup resolutions, refresh rates, layouts and such), and then you can hotkey those profiles or make shortcut icons to click or whatever other method you like for invoking them.