If you had to disable avast, it is most likely your firewall blocking avast (or your previous AV or Hosts file hijacked less likely, see below).
You should try Terminating the Web Shield rather than the whole AV.
Answer the questions relating to your firewall and try the suggestions.
Have (or did) you another AV installed in this system, if so what was it and how did you get rid of it ?
HOSTS file redirect a common malware tactic to block AV sites making it difficult to remove malware - 127.0.0.1 check your HOSTS file using notepad or a text editor of your choice, C:\WINDOWS\system32\drivers\etc\hosts or do a search for HOSTS to find it if not there. Once open you are looking for entries with avast.com on the line, you may well see other AV sites, post the contents of the hosts file.
http://en.wikipedia.org/wiki/Hosts_file