Have a look at Bob's reply in that Spyware forum discussion here;
If this DNSChanger is a rootkit then it may or may not be detected by any anti-virus scanners. Rootkits are particular forms of malware that, if carefully written, can remain hidden from AVs and anti-malware scanners unless the rootkit is specifically known and programmed into the AV or anti-malware scanners detection scans.
If you have visited that FBI site and got the green image, then that is sufficient to know you are not infected by the DNSChanger. If you are not infected I wouldn't have expected Avast to show anything in the scan re4sults anyway.
I hope that helps.
I've been following the discussion in the CNET forum that discusses the DNSchanger trojan threat that was originally posted back in Nov 2011. The main question I have is, does AVAST's virus definitions have the ability to identify this trojan? And if it does, can it also be cleaned? I have the most updated free version, and to date I have not seen that trojan show up in the results.