I always love a challenge so I went and found how to do this.

You just have to delete the log.db file.

This file is located in the Avast or Alwil Software folder of the ProgramData or Application Data folder depending on what version of Windows you are running.

You have to temporarily disable self defense in Avast and end task on all avast entries in task manager then you can delete the file.

The Avast service will restart on its own so you may need to delete the file quickly after ending task in task manager.

edit: Fixed name