Same thing happened to me. I have to shut off "Web Shield" to fix it. Just reducing its setting to "normal", doesn't work.
But, I also had a secondary symptom - I could not SEND email via Outlook Express. Receive, yes. Send, no.
Where did this Web Shield thing come from? I don't remember anything by that name before, and only noticed it yesterday while trying to track down the problem.
On my system, Avast runs alongside Zone Alarm, and Pest Patrol. Never had a problem until this "Web Shield" thing got installed.