I am an old guy now in IT (by default) and only know enough to be dangerous. I get my tech input from the 49 year old redneck/hippie techno-geek who "works" for us. I have used both Norton and McAfee subscription versions on various machines and when those expired I too used AVG based on his advice. This was in conjunction with Ad-Aware and his latest discovery, malwarebytes. All of this stuff seemed to work fine together.
Another friend (whose computers I am constantly cleaning up because for him just hitting the "on" button seems to bring in a virus) had a tech buddy recommend comodo.com, another free download. I had never heard of it, but after using it to clean up his machine for the umpteenth time I uninstalled all of the virus, spyware and malware programs on my home computer and gave it a try. First scan found 4 viruses and 2 worms that AVG and all of the other programs had missed so I am running that for the time being.
If you have a windows machine that is "legal", then you can also visit "safety.live.com" and get a free scan from Microsoft. This can go into parts of the operating system that are usually off-limits to other scans. It's easy to use and also free. Just be sure to uncheck the box that asks if you want to "share your results with Microsoft" when you are finishing the clean-up.
At any point in time Norton is better than McAfee is better than AVG, etc. All the techies I know say that all the anti-virus programs hop-scotch each other on performance based on who has just completed the most up-to-date and comprehensive virus definitions.