There's a very good series of articles at Security Focus that examines and debunks a lot of the common reasons for not using anti-Virus software.
The author, David Harley, is actually aiming at people like me who until recently though they were smart enough that they didn't need anti-Virus software. In fairness to myself, my PC did not have any viruses when I installed the software. As these articles point out it would have been a matter of time, sooner or later I would have been infected.