If you haven’t rebooted or had any other popups since that, then right click on the avast tray icon and select ‘Show last popup message.’

If this isn’t available, then try checking these files shield text files WebShield.txt and FileSystemShield.txt in the avast Report folder.

Folder locations.
C:\Documents and Settings\All Users\Application Data\AVAST Software\Avast\report\ (XP location).
C:\ProgramData\AVAST Software\Avast\report\ (Vista or later).