I recently suffered from the DNSChanger trojan. This particularly nasty piece of malware changes the DNS settings on the infected PC to a server which then redirects the web browser either to advertising or more maliciously to phishing sites. Unfortunately the virus checking software that I was using at the time - AVG Free - failed to pick up the trojan.
This failure along with a number of other gripes that I had with AVG resulted in me looking for a new free anti-virus package. In the end I decided on Avira AntiVir Personal, after having read good things about it on a number of forums. The only issue that I have with this software is that it pops-up an advertisement every time that it updates, but this is a small price to pay. Avira has something of a reputation for finding a lot of false positive, which is when a safe file is mistakenly identified as a virus; however, this is preferable to the opposite problem, when the software fails to identify an infected file.
I finally managed to remove the trojan from my PC using the excellent Malwarebytes' Anti-Malware. A version of this software is available for free that allows you to scan your drives for any infections. You can pay for another version that offers real-time protection from malware.