Is there a hotkey to close all open windows simultaneously?
No. But you can minimize all windows simultaneously by pressing Windows-key+D. This hotkey does not work in Windows® 95 or NT. In these older operating systems, press Windows-key+M. If your keyboard does not have a Windows-key, press Ctrl + Esc followed by Alt+M. Another approach is to Shut down Windows without closing any applications or windows. If documents need to be saved, Windows® will prompt you to do so before closing down.