I ran the offline installer for Avast Free 17.4.2294. I opted for a custom installation and selected my choices. Where is the INSTALL button? It was nowhere to be seen. The only way to exit the dialog box was to click the window close X button.
I also had the offline installer for Avast Free 12.3.2280 and this did provide me with an INSTALL button. Problem solved after updating to the latest version.