I've been without anti-virus software for around five years. The day I removed it from my work computer, my productivity went up at least 20 percent, due mostly to the poor thing not choking itself to death with constant and untimely scans and update procedures (updates at least a weekly occurrence requiring something like a 50meg download each time).
Most of these scans also happen (unavoidably) on startup - which is kind of an inappropriate time, considering I shut it down the night before. I promise nothing has happened in this time, dear programmers, and when I get to work I expect to be able to work and use programs, not be faced with a little twirly icon in the corner as the only evidence of life for the next half hour.
In fact, on occasion, mister anti-virus was scanning and updating so hard he couldn't even twirl his own icon. After the fourth or fifth time that happened, and a few therapy sessions later, I realised I was in fact on candid camera. Off with his head.
I also have not had this software on my personal laptop since I bought it new two years ago and have never been happier. I just don't do anything too stupid. Now I'm not saying that anti-virus companies are the ones inventing viruses, or at the very least inducing some sort of low-level panic - I'm not saying that. I mean I'm just saying...