Download the avast! Uninstall Utility,
find it here and save it to your HDD. Now uninstall, reboot, run the uninstall utility, reboot install the latest version, reboot.
You can switch to manual updates, Program Settings, Update (Basic).
Or you could have it check for updates and 'ask' that way you would be aware that an update is available, just ignore the notification and do it later.
You could also delay the update by a few minutes, what is your internet connection, dial-up, etc. ?
However, the avast auto updates shouldn't be slow the signature file updates are incremental and are typically about 25-50 KB or less unless you haven't updated in a while.
Program updates are different:
1. they are set to ask by default so shouldn't update automatically.
2. the update cycle is spread over 7 days to balance server load as they may well be larger.
There has recently been a program update which may be what you have experienced.