What is your firewall ?
Does it allow avast.setup internet access ?

  • If it does delete the entry for it and do a manual update, this should force the firewall to ask permission again.

You say your OS is MS Windows 8, since the latest windows OS is windows 7 SP1, that is obviously incorrect ?

  • Do you connect to the internet using a proxy (web accelerator, ISP, etc.) if so you would have to co-ordinate that with the web shield proxy ?
  • Try this also if the above doesn’t make a difference - From the avastUI, Settings, Updates, Proxy Settings, set it to Direct connection (no proxy), see image.