I don't use antivirus software. My old computer, which I never used antivirus on, lasted 6 years with no problems (it's still fine, but I upgraded because it was getting out of date).
I'm convinced that all the popular anti-virus programs treat other files as viruses, or install extra viruses on your computer, just to make it seem like you need their product more. On my parent's computer, for example, on which they have McAfee, they had 98 viruses in one month. All my parents do is use email and read the news.