To delay the update of avast into a permanent internet connection (DSL), just change (or add) AlwaysConnectedWaitSeconds value into the [InetWD] section of avast5.ini file.
You could set the number of seconds to wait before attempting to update. The default is 30 but it may not be enough in some DSL systems. In this case, you can try increasing this e.g. to 60 (1 minute). That should be enough.