It's my experience that anti-virus solutions don't offer any real advantage anymore. They have become so bloated and take so many system resources, I don't even bother running even the free ones anymore.
Viruses sent through email are completely non-existent, all major providers are blocking or screening these, so these don't even make it to your computer. There is so much variation with the anti-virus products as a whole, it's rare the product actually catches a virus. There are so many times that I work on a computer that is infected, it has a anti-virus product on it, yet use a completely different product to clean it. Are they better than nothing, sure, but it's Russian Roulette if you have the correct anti-virus product to catch the virus is coming at you. It's my feeling that to be truly protected, you'd have to run three of them at once to prevent the failure of one (which you can't do).
The better defense is being very proactive with your updates and run a secure browser. Adobe's Flash has horrible security holes found in it constantly. So many websites run Flash and all it takes is an exploit, old Flash version, and that nasty is on your computer, regardless of your anti-virus product. Same can be said visiting a very normal safe website, yet it pulls an advertisement from an infected machine that takes advantage of a security hole in your browser, infected. Almost all of the garbage out there now comes in these forms, and I just don't see anti-virus products helping. I'd rather have my computer running fast all the time, instead of reduced speed with a slim chance of it actually catching something.