Close All Windows is a simple application that forces all your open windows to quit immediately, and the Addictive Tips blog points out that you can also pin it to the Windows 7 taskbar for quick access.
All you really have to do is download the application, save it to a permanent location somewhere on your drive, and then pin it to the taskbar using a simple right-click -> Pin to Taskbar — just make sure that you don't double-click the application or all your windows will immediately close. Unlike some of the other similar applications we've written about, this one has no interface, but it has a higher resolution icon that looks good pinned to the taskbar. If you are looking for a way to immediately close all your running applications, this could do the trick nicely.
Close All Windows is a free download and works on all versions of Windows, but it will need to be pinned to the Quick Launch instead of the taskbar in XP or Vista.