Of course it can but I feel it would be unwise to do so, I'm a dial-up user and all I do is delay the update check to 10 after I connect (this allows me to quickly connect grab my email or something else without triggering an update check).
The avast updates are unlike anything you have ever used, the VPS (virus signatures) Updates are incremental and can be measured in KBs rather than MBs with the duration normally measured in seconds and not minutes (even on dial-up) so there most certainly is no fight for a share of bandwidth.
The avast auto update also has another trick up its sleeve it also restricts the amount CPU% it take up so it doesn't throttle what you are doing, so it is very much in the background whilst you continue with whatever it is your doing.
A manual update has no such throttling as the auto update since you initiated it.
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:
Dial-up connections, add this line:
RASWaitSeconds=600[InetWD]
RASWaitSeconds=600
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 ten minutes, you could try that and adjust if required, 300, 480, etc.
