There are just too many choices. And who is to say a free antivirus wouldn't work just as good as a paid for version. My opinion is that all anti-virus software products are hit n' miss when it comes to detecting and removing a virus. However, I do think you should install an antivirus from a reputable vendor. After all It's your first line of defense. If your internet service provider includes an anti-virus software at no charge the decision maybe easy. Sandhill Telephone Cooperative offers Trend Micro Internet Security at not charge. And Time Warner Cable offers McAfee at no charge. Most important, make sure you install something.
Who truly knows which is best? But here are my picks based on experience:
Paid for versions: Trend Micro, Norton Anti-virus (I don't think the Norton 360 version is necessary)
Free versions: Avast, Microsoft Security Essentials, AVG
Note: Microsoft Security Essentials is called Windows Defender in Windows 8. It is also built-in.