I have heard many people say "I am using Avast.... I am using Antivir, haven't got a single virus so far". But the fact is these two (Avast and Avira Antivir) are not that good. You guys have to check your systems with different Antivirus programs (like Nod32, Kaspersky...) from time to time (of course, you'd better run only one ANTIVIRUS at a time).
Most of the Antivirus programs' efficiency depends on the databases of their virus definitions, and how often they are updated. That means they can only find out a virus if that virus is already in their virus database list. This kind of Antivirus programs are almost useless since they can't find a new virus that isn't in their database. Using this kind of Antivirus programs means taking chances with the security of your PC and your personal and sensitive data.
But some antivirus programs like Nod32 (ESET), Kaspersky, Prevx use different technology (like monitoring the status and work of a process) along with virus signature database help. So, to conclude it, using this kind of programs increases the security of your PC.