I have SOLVED the problem by "ticking" the box for "permanently connected to the Internet" ( Even though - as mentioned - the computer is NOT "permanently" connected because it connects wirelessly only when in range of a wireless conection point ).
This setting just say to avast the way the user connected. If nothing is checked, avast should check for a connection as well. It's just to 'speed up' the update process searching for a specific kind of connection.
avast checks if there is an available connection each 40 seconds.
If there isn't, wait more 40 seconds to check. Checking does not take more than one second and, of course, does not use the Internet band.
If there is a connection, check for an update. If there is not any new file to download, wait 4 hours to start checking again. If there is an available update, start it and install it. Again, wait 4 hours to check the next time.
Some of this settings could be configurated different, I posted the default values of the avast4.ini file. The updates are incremental.
If a system is running with a permanent broadband connection then:
With the "Dial-up" modem option checked and with the "Always connected" option unchecked
avast will never check for automatic updates if the user has a permanent connection and let it off. Sorry, my first post was wrong and I've edited it.