The worst of such softwares I've seen is Symantec's Norton antivirus. It's not free, it's been known to allow viruses come through because the controls and expiration messages are confusing.
The freewares are better. First, there is Avast. But then, by experience, AVG is better than Avast in the sense that I've gotten rid of a type of virus which couldn't be detected by Avast.
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