I have recently found that Zone Alarm (and some other firewalls) sometimes blocks Avast from scanning email by blocking certain ports on machines using the 127.0.0.1 interface. This interface is also common to AVG Antivirus, and Norton may still use this type of routing, but it was used more predominantly in older versions of Norton Antivirus (and it sometimes caused difficulty for folks using Outlook Express).
To fix my problem, I went to the firewall settings. In the "trusted" and the "internet" zones I check-marked the "allow incoming tcp ports".
In the "Ports" box, I typed "5100, 5101". These are the ports that need opening. More ports counting up from there may also be blocked, depending on the number of POP & SMTP mail servers used in the individual configuration. You also may need to go into the Zone Alarm "Program Control" section and make sure "access" settings are enabled for the respective programs. If the Avast program is absent, click the "add" button near the bottom right and browse to the Avast folder. Select and Open the Avast program along with the "ashmaisv.exe" icon. If you still have problems, reboot the machine and start the email wizard from the Windows Start Menu and select the manual settings. Press the next button, and when you get to the email programs selection, make sure that you check "select all". Finish-up and you should be in business! If this does not work, try uninstalling the firewall and Avast, reinstalling and configuring Avast without any firewalls installed, then reinstall your firewall. All of these solutions have worked for me. I have read some negative responses to my first, hastily-written suggestions, but I have not read any other suggestions that have worked for my initial problem. I like Avast, and feel it is a benefit to the antivirus market. Keep up the good work!