The avast.setup is the avast update process so it is getting in early in your boot as a connection is available.
You can delay that check to allow your system to completely finish booting before it starts it 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 (probably broadband) enter one or other of the Bold lines below):
Dial-up connections, add this line:
RASWaitSeconds=120Broadband connections, add this line:
AlwaysConnectedWaitSeconds=120[InetWD] AlwaysConnectedWaitSeconds=120 Or RASWaitSeconds=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. If your system normally boots a little slowly then 180 seconds or more may be better.