Guess what, they already do, on systems with XP or higher they have a hack to restrict CPU (older OSes haven't that functionality) on the auto VPS updates, this isn't used on manual updates as it is assumed the user is ready to proceed.
So you have to monitor what actually runs on start-up you can also delay the update check in the avast4.ini file.
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 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.
Me on dial-up I delay mine by 600 sec (10 minutes), which allows me to quickly jump on-line collect mail and get off without initiating an update check.