I hate this "I've been running without AV for ages, and I never saw any viruses" argument. Of course you didn't. Making you aware hasn't been the motive for a long, long time.
It's virtually impossible to run a Windows (or Linux, or Mac) installation with the usual suspects--Java, Flash, Adobe Reader, etc.--without being exposed to good old, non-targeted malware. Take into account that most of it is distributed from "good" sites, and "if I don't see it, it must mean there's nothing there" and "I never go to any risky sites" prove pretty silly.
I'm not saying that AV is the best solution, or even a good one--indeed it can even be what contains the vulnerabilities used to take over a machine--but there is a reason it exists. Let's not pretend that AV solves all ones problems, but let's also not pretend that it's completely ineffective. It's only ineffective if there really is no other (probable) way for things it detects to get through--which there is on most desktop operating systems unless you (manually) go to great lengths to isolate the different things you do. There are ways to make AV moot, but it rarely comes built-in or without user overhead/experience requirements. (Ironically, in Windows 8 anti-virus comes built-in.)