Why does everyone getting so worked up about this pop-up ads?
You're using Avast Free.
Avast offer you Google Chrome upon installation (not some annoying toolbar).
Avast shows ad in it's UI (which most normal user rarely see).
Avast shows ad in it's small pop-up once or twice (at least the pop-up is relevant and not filling up your screen or stealing your window focus)
I've tried other free antivirus from other big name vendor (Avira, Panda, AVG, and MSE) and end up choosing Avast free over others. Other antivirus offering and ads are worse than what Avast does. MSE free from offers and ads? Sure it does, however I'd still choose Avast over MSE anytime (as long as Avast keep improving it's performance and stay light)
Being an antivirus company is a business, and it's need to generate profit to stay healthy, expanding the company, and improving it's service quality.
Be thankful of what you've been given and remember that nothing it's truly free in this kind of world.