An Estonian company called Rove Digital was busted last November. Why? Because it was a front for the ad-fraud DNSChanger botnet. And ever since November, the USA's FBI has been responsible for the substitute DNS servers designed to keep compromised computers from being disconnected (and causing support call chaos).
F-Secure Labs also has something to offer: DNS Check.
It's a script-based tool that can be used to reset problematic DNS settings.
DNS Check will scan to determine if the computer's DNS is configured to use the botnet's servers (now the FBI's) and can be used to reset default settings to DHCP, OpenDNS, or Google DNS.