Delay the auto update check.
You need to edit (using notepad) the
[InetWD] section of the C:\Program Files\Alwil Software\Avast4\Data\avast4.ini file and add the following line (depending on your connection type assumed as broadband):
Broadband connections, add this line:
AlwaysConnectedWaitSeconds=120[InetWD]
AlwaysConnectedWaitSeconds=120
When complete save the changes, avast's self-defence module will ask for confirmation, etc. answer Yes.
The figure is seconds and the above equates to two minutes you could try that and adjust upwards if required, 180, 240, etc.