for free avg is pretty good
myself i use ESET smart security (not free but that never stopped anyone getting it free)

Microsoft Security Essentials or Avast is the way to go.

Both are free and are great AV software.

Plus they both use very little RAM unlike others. (Norton, McAfee, BitDefender, etc.)

AVG, free and never had a problem for me, been using it for years try it out

-JoshRobo98

The number one thing to prevent malicious programs from getting on your computer is using Common Sense 2012.

If you insist on an AV program, ESET catches a lot of stuff. If you pay for ESET, I can pretty much guarantee your computer is safe.

Otherwise, MSE, Avira, and Malwarebytes all have worked fine for me.

I would get AVG. There is a free version, but if you like getting the full version for free then here is a link to Youtube telling you how to do so:

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Basically go to the AVG website, download and install the trial, and then use one of the license keys in the description of the video to activate it, and have it forever.