How do the makers of Avast make money and stay in business?

Referring to the anti-virus product Avast:
The basic version is free, and I would think that is all most people would need. I can’t imagine enough people buying the upgrades with the additional bells and whistles to keep them afloat.
So how do they do it? Do they really keep the product updated and up-to-the-minute? I think this would be critical in an anti-virus product.