I have EXACTLY the same problem. I actually had Avast working fine. After installing ADSL, Avast was not updating virus definitions. I couldn’t figure out why and decided to reinstall. Once I had removed it, I realised that it may be due to blocks in the firewall; it was. So I installed again and now have exactly the same issue you describe here.

No matter what I do, I am unable to rectify. Will keep tracking this forum to see if someone comes up with an answer.