Try switching the proxy settings from “Auto-detect” to “No proxy” (I’m assuming you’re not connecting to the Internet via proxy).

Left click the avast icon > Settings
Update (Basic) > Details > Proxy

If this does not help, can you try to repair your installation?
Go to Control Panel > Add/Remove programs > avast! antivirus > Remove
Then choose Repair function in the popup window (Repair).
You must be connected to the internet while repairing.