If the header information contains those lines then your inbound email is being scanned, so I can't understand why the avast a icon isn't rotating. If you double click the avast icon you should be able to see the avast! On-Access Scanner window, select the Internet Mail provider from the list of providers, there is a Scanned count: and Last scanned: items these should increase and change respectively, do they.
I asked for the server domain info because I want to know what the email is likely to be, if it were sympatico, etc. then the email might be using encryption. This gives me clues to try and help you without it I'm in the dark and guessing. Outlook Express, Tools, Accounts (select one), Properties, Server tab. Now you have the image I posted what are the details ?
Do you use Hotmail/MSN, Yahoo or any other web based email ?
Do you use gmail ?