Avira Free Antivirus 2013
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Full user review
"The best free antivirus"
Best detection rate of all free AV's, also confirmed by dedicated AV tests. Extremely low on false detections. Quite lean on resources. Less chatty than its competition regarding the file analysis - actually totally silent if it isn't absolutely sure something presents a risk.
New malware definitions can sometimes get late compared to others, or perhaps this is because its heuristics aren't so trigger-happy as the competition, sometimes it can happen a very new malware slips its radar, doesn't happen often though. Displaying ads for its full version can be annoying, but it's not too often and it's easily dismissed.
This is the only free AV I found works as seriously as a paid products, in fact the only reason I find its ads for the paid version annoying is that I don't feel the need for the upgrade! This one works better than other free solutions and is comparable to the commercial versions. I never feel a need for a "full" AV when using this one.