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).
Boot.
Updates > Proxy Settings > Direct connection (no proxy)
Then Manually update.
That fixed it for me. Of course, a few days later I had to switch back to “Auto detect (use Internet Explorer settings)” which is where my settings where to begin with
because I started getting the same problem again.
Seems to be working fine now. Not sure what the underlying issue is with this, but at least that should help you keep your Avast! up-to-date until it’s sorted out