For petes sakes people, this has nothing to do with registering.
I have always been registered, and still is, and there's a giant pop-up in the corner once per day or so, offering me to buy their products.
And you can drop the ridiculous nonsense about how people shouldn't complain about a free product. That pop-up is in nobody's interest, and will only driver the users away from Avast. It's not going to make people buy it. It's just going to drive them away, which in the long run, means the fewer people buy the product.
This is not a once in a while information about a product. It's frequent advertising, and it's not ok.