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Microsoft Security Essentials is Microsoft's free antivirus software. This software is very effective at staying updated on the latest virus definitions and eliminating malicious threats. It is important to note that this software is for PCs ONLY. You can download this for free from Microsoft's website here: http://windows.microsoft.com/en-us/windows/security-essentials-download
MalwareBytes
MalwareBytes is another free software that's great at removing viruses and other malicious programs. You can download this software here: http://www.malwarebytes.org/