Yup! Avast 9 messes up Opera 12. Ironically, it was the misrendering of this forum which first alerted me. Opera 12 works perfectly with Avast 8.0.1497, as does everything else.

Please do not presume that the latest is better. The final insult is the yellow warning triangle in the Avast system tray icon. That is surely to warn of more important things than the user having consciously declined the use of the later version. If it cannot be suppressed, I will perforce put up with it.