As far as i know, free Anti-Virus are as good as commercial ones.
But... I don't use Anti-Virus, so don't trust me so much
The anti-virus companies have been terrorizing us for years, telling us that our computers are going to fail if we don't use their software. But in fact, weakest part of the system is the user.
Internet is a reflect of real world and as we walk carefully on the street, we must be careful with Internet. I've been without antivirus for many years, I have had no problems and only by don't opening 'strange' things.