I am using GPRS, command to establish dial-up connection is in my StartUp folder, so connection is made just when I log in. In avast Settings in Updates(Connections) both boxes are unchecked.
If you use a dial-up, I do suggest you select the first option in that settings page. This will allow the behavior described by Vlk: avast does not even attempt to do the update if an Internet connection is not available…. 