This (almost always) means that something is preventing avast from setting the intercepts it needs for email and NNTP scanning. In most cases this is because a previous anti-virus product has not been properly removed and is still active in the system.
Is this a new installation of avast? If so, what was the previous program and how was it removed?
There is a chance that Comodo is blocking that request without asking the user…
Sometimes, removing the ashMaiSv.exe (avast mail provider) and the email program (and spam killer application, if any) from the allowed programs at Comodo and then trying to download emails again could make it work (of course, allow connection when Comodo asks for).