On one PC, the 2015.10.2.2214 update of Avast Free Antivirus blocked DNS. Windows 7 SP1 32-bit claims no Internet access and nslookup times out. There is nothing wrong with the DNS configuration or servers. Switching off web shield makes DNS work. Even with the web shield on, it is possible to access web sites via IP address, and it's possible to tracert -d to the DNS server. I tried using Google's 8.8.8.8 and 8.8.4.4, which are certainly legitimate DNS servers.
It was interesting that after the update I did not get the Google Chrome offer dialog. Uninstalling, running avastclear and reinstalling with default settings does not help. After a reinstall, I couldn't uninstall normally, because it said the installer is running. I had to reboot into safe mode to uninstall. I wonder if something is going wrong at the end of the installation process. Going back to 2015.10.0.2208 fixes the problem.
That PC has absolutely nothing else installed besides Windows 7, SP1, drivers and Avast. The firewall is configured for a public network. Maybe some of this is triggering a bug?
Is there an Avast feature relating to DNS which can be disabled? I read that Secure DNS was introduced recently, but that's apparently not available in the free version, and I see no setting for turning it off.