What OS are you using?
Have you checked to see if the emails are being scanned, there should be something in the email headers.
X-Antivirus: avast! (VPS 0542-5, 21/10/2005), Inbound message
X-Antivirus-Status: Clean
Or check the Scanned Count in the Internet Mail provider details.
Or does the avast icon rotate when you are receiving email.
Or does a little icon appear on the system tray when you are receiving email?
You may need to set it up manually, but check the above first.
The server address for both POP and SMTP should be set to 127.0.0.1
The POP server name set to: username#pop.isp.com (use the actual username and ISP info)
and... SMTP server name to: username#smtp.isp.com
You can go into the Help file within the Avast Scanner and look up Manual configuration. It will take you through it step by step. It is not hard to do.
Or see this thread for further information and advice
User's FAQ - Mail Protection.