Avast Free Antivirus
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Full user review
"Does what it should, doesn't do quite everything"
Automatically and quickly downloads new definitions every startup, if not more often; let's you know when there's a problem, not in-your-face intrusive as some apps are. Small processor footprint, doesn't seem to affect system response speeds at all.
Doesn't protect from email phishing and fakie website scams, but it's free. The paid version gives the additional protection this free version lacks. You must register the product to get an activation code, so not anonymous.
With this application installed, I have yet to be infected or be the victim of a trojan or other attack. While it is part of a greater protection strategy, such as being behind a hardware firewall and sticking to safe web practices, Avast Free Antivirus is great at doing its job without bogging your PC down or bugging you constantly. Worth every penny it costs and then a lot more.