cnw,
Not hard at all. When you have the Avast interface open,
it allows you to "local drive, removable media and specific files/folders.
If you click on the "scan local drive icon", a small dialog box opens with the heading "Thorough" and a small box "Archives".
If you check the small box, archived files will be scanned each time Avast runs the scanner.
Simple!
Welcome to the forum.