Really, that seems strange, with all your tools you have.
Either way, back to topic.
List of good, free anti-crap programs.
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AVG
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Comodo Internet Security
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Microsoft Security Essentials
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Avast
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MalwareBytes - if you can afford it, buy the pro version.
Remember, just because you have a "good" anti-virus, doesn't mean it will stop everything, don't go downloading shoddy files, if you think someone looks a bit weird, scan it before you run it. It may seem like a pain, but it will be worth it if you find it's a virus and it would have done some damage to your computer.
Anti-virus programs generally only stop viruses, you still need to have a malware check, or leave malwarebytes running all the time, it will find almost everything. Sometimes you have to do some manual work,, but it will find nearly everything.
Even paid programs sometimes don't catch everything.
Also, only ever run ONE anti-virus, or they will conflict with each other and might end up catching nothing with it. Two condoms is not always better than one. Might make you feel safe, but it really isn't any safer, it just uses more resources, and reduces pleasure.