Hi adddm,
The Boot-Time scan generates a report file called "aswBoot.txt" which is found in the folder Bob mentioned - C:\ProgramData\Avast Software\Avast\report\ Please note, this is the \ProgramData\ folder, not the \Program Files\ folder (where the log.dll file is located).
The other scan reports should be generated in the C:\ProgramData\AVAST Software\Avast\report folder, but you may have to enable this option first. To do this, please open the "Virus Scans" screen > click the settings 'cog' icon in the top right of the screen > scroll down to the bottom of the "Full Virus Scan" tab and check the box next to "Generate report file".
You can enable the report files in the same manner for each of the other scan types on their respective tabs except for the Boot-Time scan (which will always generate the log file).