gfreed,
what was the type of file that was being scanned (and were you trying to get avast to scan file)?
You may want in Thunderbird to go to Tools > Options > General tab and use the "Check Now" button to make sure that Thunderbird is set as your default mail client. MS has a nasty habit of un-setting this especially after performing any MS Office updates or using Outlook.
If you are not using Outlook at all then you are correct in thinking that you do not need to be running the avast Outlook/Exchange provider. The avast Internet Mail provider is the one that covers Thunderbird and Vista's Windows Mail.