Thanks David and Gopher for your replies
The problem was only occurring with firefox. I looked up browser.download.manager.scanWhenDone figured out how to open the firefox configuration file (typing about:config in the location/url box & hitting enter), searched for and found Avast referenced under 'downbar.function.virusLoc'.
So I reset that and it now says:
I downloaded a file and Avast didn't come up!
but I got message about the virus program not being listed properly in the download settings.
I have the 'Download Status Bar' addon, so I unticked the 'Manually scan downloads' box in the option settings for that and now I don't get an error notification or Avast window so I am *very* happy!