The System here is XP Pro SP3. Avast 4.8 always logged a confirmation message in the Antivirus Windows Event Log, as did Avast 5 the first time it updated.
In 4.8 an icon, similar to the yellow Microsoft one which always accompanies updates (so that one knows what is going on - I am on a mobile phone connection paying by the minute and sometimes log off before Avast has finished updating, hence the useful update icon). There used to be a setting to to activate this feature but I cannot find it in the latest version.