Ideas: Double check in the Avast Mail provider settings under "Internet Mail>customize" then the "redirect" tab, that the POP port is set to 110, the SMTP to 25. (Default).
Run the
Appropriate AVG removal tool (Either the 32 or the 64 bit tools) to remove any AVG remnants you may have.
Remnants of old AV's can cause problems some time after a new AV has been installed. Kernel level drivers, etc.
Make sure the process "ashMaiSv.exe" is allowed in your firewall.
Don't happen to use ZoneAlarm, do you?