hi P-squared,
I'm assuming you have got 8.0.1483 stable avast! free installed (you didn't say).
Open the avast! gui (program window) and click Settings in upper right corner: Settings>Popups.
There you can change the popup duration time to 1 second if need be. Unfortunately, since the free version is supported by advertising, you may still have to allow the pop-up, but at least it will now come up and disappear quickly. I know, this can be very annoying, but I would rather have the avast! protection in place in spite of this behavior.
See attached .jpeg below for more information. 20 seconds is the default setting for all pop-ups; all popups can be changed to greater or lessor times if you wish.
Purchasing a paid version will remove the ad popups, but not saying you should, btw, because you get the same virus engine for protection in free as in the paid version...
EDIT: If you can, choose "Show last popup message" and screenshot the offending popup by right-clicking the avast! icon in the system tray and attaching that .jpeg in your next reply.