Close all open windows and applications at once, even selecting what to leave open, with CloseWin
CloseWin is a small utility which closes all open windows with a press of a button.
The program will also let you close only windows you select by specifying containing text.
No need to install; extract the files in a folder and run the executable.
CloseAll (included in the same download) is a similar but simpler utility. When you run CloseAll, it will close immediately (without asking for confirmation) all open windows.