Keyboard shortcut: To minimize all windows currently open (show the desktop), hit WINDOWS+M. To remember this, think of the ‘M’ in the shortcut as the letter that minimize starts with.
To restore all windows, hit WINDOWS+SHIFT+M. Windows that are minimized are not closed, they can be restored by clicking on the window’s button on the taskbar.
In Windows 7 you can minimize all windows (show the desktop) by moving the mouse pointer to the rightmost edge of the taskbar and clicking. Click again to restore the windows.