Cannot connect to the internet after Avast installation

Hi,
I am not able to access the internet after installing Avast Free Antivirus. I have tried all browsers installed on my computer (Chrome, IE and Mozilla). All web pages reply with “cannot access the internet”. When I disable the antivirus I can connect to the internet but this is not a solution because the idea is to secure my online environment. What should I do? What is causing such an error and what other free antivirus software are available?
