Worth mentioning that location description is a bit disingenuous even if the OP found it.

The log is not found, as it may have seemed to be inferred, in the main Programs > Avast folder but, maybe dependent on the Windows version, in Documents and Settings > All Users > Application Data > Avast.

The Application Data folder is usually hidden by default so you’ll have to click on the Folders tab, select the View tab and change the option so that it shows hidden folders.