I use AVG Internet Secutity. Mainly because it was cheap at the time (£15 for the first year w/ unlimited installs) and it does work. It picked up one virus a couple months ago, and its been fine since. Amazingly, It's particularly light on my poor old Dell, which struggles when playing 720p YouTube, so I'm happy with paying the horrific renewal prices that AVG put into place. Every time I scan I do a deep scan, which helps pick up those nasty viruses.
Of course, I do recommend AVG Free Antivirus if you can't afford Internet Security. And, if you don't like AVG at all, try Avast. It is light on older systems, such as my Dell, and can pick up a lot of viruses and malware if you have a badly infected system. Of course, when using free Antivirus, or any at all, get Malwarebytes. I don't use it because it puts a big strain on the CPU. I didn't intend to ramble on like this...