Avast is excellent - I use it on all my windows based kit (and on my android phone too).
They make money because some people upgrade to the 'paid for' product which has more features - the business (multi user) version is also a 'paid for' product.
They also get a kick-back from google because they encourage you to install/use Chrome.
Avast antivirus is a great free tool which protects your PC from viruses and online threats. It is totally free but if you need more features like parental control it's better to purchase the pro version. Personally I prefer Avast because it gives a great protection and it does not slow down my PC.and thy make money mainly by selling to the buyers who has used there product free.it is some kind of promotion for them.if you want more details about features of avast free here you can learn more