Hmm. I think Ubuntu Linux is the best "free" anti-virus software.

Why? Viruses and malwares seemed to be a Windows plague and attracted to Windows. Ever since I turned my PC into a dual boot machine, I'm connecting to the internet via Linux. And during that time, I'm not even thinking about getting infected by viruses and malwares.
Anyway, I'm checking once in a while to verify if my assumptions still hold. Since I last checked, there's not much viruses rampaging the Linux world.