I did finally notice that I had not downloaded a deb file. I followed your link and had no idea on how to run the terminal program… but figured that out:

click on dash home icon, top left.

type in terminal

run it = purple screen
at prompt: :~S

type avast

I was prompted for the code I was email and using the mouse to copy and paste worked.

— The avast program the scanned my machine in a couple of minutes and found nothing.