What is your firewall ?
What wireless ISP ?
How is the connection made to that ISP (modem, router, etc.), excuse my ignorance, I haven't use one ?
Have you checked your avast Program Settings, Update (Connections), ensure it is set correctly ?
Also check the Proxy, button and ensure that these settings reflect your circumstances, Direct connection (no proxy) seems to suit most people, unless their ISP uses a proxy).