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avast! Free Antivirus
Expect great all-around protection against viruses and trojans. 
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AntiVir Free Version
Lightweight and solid. Good ratings. Worth looking into. 
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AVG Antivirus Free Edition
Not as good as Avast or Avira but with considerably less bloat. The most popular free antivirus. 
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Microsoft Security Essentials
Simple, lightweight and competent protection suite. 
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Comodo Internet Security
Free security suite, includes both a firewall and anti-virus protection. 
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