If as you said, "The folders had been set up and operating prior to the anti virus install", then I probably wouldn't go looking for other causes. If an uninstall of the antivirus suite returns your network to operating as before, then I would assume that you should contact either McAfee or AT&T to resolve this issue. Personally, I use Link Removed due to 404 Error and the built in Windows 7 firewall and feel quite safe. Of course those types of decisions are yours to make.