Firstly the avast.setup file isn't a permanent file, it is created to handle the avast update and is deleted on completion, so it would be somewhat difficult to rename, even if avast's self-defence module wasn't there.
What is happening is your system has an internet connection and avast detects that and checks to see if there are any. Boot time can be somewhat busy, so the avast.setup might not be able to get cpu time and could tie things up depending on what you have starting on boot.
You could try delaying the auto update check until things have settled down after boot.
You need to edit 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 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
The figure is seconds and the above equates to two minutes, you could try that and adjust upwards if required, 180, 240, etc.