..When Avast trys to access the net, one of the Comodo popups will be something like "do you want to allow 127.0.0.1 TCP out etc etc". Click allow and then Avast is granted loopback. Comodo allows loopback per application, not globally and it will always ask if its needed.
..Zonealarm just allows/trusts loopback globally for anything which isn't very secure.
Originally Avast started before Comodo, so Comodo could not give you this popup. So loopback was not allowed for Avast. So Avast crashed.
To get this popup. Just right click Avast icon, click Stop On Access Protection, wait a few seconds, then restart On Access Protection. Comodo will then popup an alert as Avast starts and/or when your online, just allow anything to do with avast and click remember.