I have exactly the same problem in my 3 PC's
You can logon batcat but after waiting for about 7-10 minutes. Try it

Fixer thx but your proposition doesn't work for me.
I search the forums for solution but I guess avast developers have to do it.
I want to add more details to the configuration batman wrote.
Win XP Pro SP2 with all the recent updates from microsoft.
There is also installed the latest MSN Messenger 7.0 wich may be the problem (MSN explorer tries to open messenger too)
No software firewalls enabled (not even XP's)
BUT
I'm behind a simple ethernet ADSL router (alcatel-Thomson speedtouch 510i)
I hope this is fixed soon. I don't dare to deactivate webshield, So i better wait those 7-10 minutes to sign in. Hmm forgot something that may be critical: If you sign in like this (after waiting 10 mins) you cant see your favorites in msn explorer, but if you deactivate avast on access protection and sign in immediatelly you have your favorites back.