Microsoft Security Essentials is, as far as I know, completely ad-free.
While it may be reasonable for a business offering a free product to include an occasional ad, if your competition is also free and doesn't have ads, customers are going to consider migrating.
I like Avast a lot and have used it for years and recommended it to people, but if these ads become a regular thing, I am going to consider switching my anti-virus software.
The first ad, I ignored. The second one, I decided to look up this forum to see what's going on and register to post. The third one, who knows? I may switch to a different company. I'm thinking about switching for the first time in years.
It used to be that Avast's main competition either charged an annual fee or offered an ad-riddled free product. In that environment, if Avast had started inserting these advertisements, it probably still would have been the best of the bunch. MSE has changed the game, though. If they are free and have no ads, why would I use a product that is free and does have ads? Maybe inertia -- but when the annual registration time comes around, or if the ads get to be too much -- that reason will no longer exist.