thanacha,
I would suspect that the settings of MS have defaulted to the original settings and may have to be manually configured.
Take a look at the MS 2003 settings for the user account.
The servers for POP3 and SMTP should be set to 127.0.0.1
The username should have been modified to reflect the username from your ISP modified to something like:
username#pop3server.isp.com
username#smtpserver.isp.com
If the setting show the standard username format such as
username.isp.com..............then,
the settings are not configured for Avast.
You can open up the Avast help file and look for Manually configuring Avast.
Good luck