Use your head. Most good antivirus programs don't have corny names like Antivirus 2010 or WebProtector 9000.
Download stuff you can trust, like AVG, Panda (although AVG would be a better choice IMO), and buy larger brands like kaspersky or norton.
If it is downloaded by a lot of people, read the feedback that is not on their site. Do searches for opinions on forums to make sure.