AV is a joke. People get viruses because they're dumb enough to click on every single link they see while browsing porn websites and whathave you. I have never actively used any form of virus software, every 12 months or so i install the at the time most popular software on the market, and it turns up bugger all. If you have any degree of common sense you should not have any need for AV software.
While there are some viruses that are unavoidable at times, from all my experience with computers, folk with viruses, well they deserve to have viruses because they're dumb as shit and shouldn't be allowed infront of a computer without supervision
I don't know if it's as common place now as it used to be, but the vast majority of viruses friends would end up with were due to p2p downloading, they'd see a film or game or song and think oh awesomeee, not taking two seconds to note that it was a 25kb file as opposed to the 700mb a film might typically be.