Githeko, are you using a proxy filter?
If not, check your proxy settings into avast!: Left click the avast icon > Settings > Update (Basic) > Details > Proxy (like Cloussau said).
Make sure your browser is not set to "Work Offline" (this option is generally in the File menu). If it doesn't help, try switching the proxy settings from "Auto-detect" to "No proxy" (I'm assuming you're not connecting to the Internet via proxy).