Right click on the Avast Icon in the system tray to bring up the Avast console.
Click on the "Local Disks" option (top right part of console) hover over the three icons on the right of console (you will see "Local Disks").
Click on icon and then select "Scan archive files" by clicking on the check box.
Archive's will be scanned with this selected (and will take longer to scan as you probably know).