I have been running my Windows PC without any anti-virus, anti spyware, adware and pretty much anti everything for about a few years now. I told this my buddies and they think i'm stupid or something. Unsurprisingly, this decision never came to haunt me.
The primary reason I do not use Anti (insert windows flaw here) is because of the performance issues. Seriously, every AV (Anti-virus and the like) software I ever tried has eaten up so much of my CPU and RAM I figured it was not worth it. Half of the Av software is in itself a virus. Remember good ol' Norton? I remember spending countless hours trying to completely remove the thing, on later to find out Norton had complaints of people unable to completely remove the software. They made a tool that removes it, but by then, your PC was so screwed up and slow that it wasn't even worth spending the time to remove the thing. You might as well just do a destructive reformat (I like to call it a 'nuke').
You still probably think I'm crazy, and so be it. I don't get viruses. I always wonder how to get a virus anyway. Yes, you read that right. I hear people talking about how they spend countless hours trying to remove a virus, installing countless AV software each promising to remove it. I seriously think people get viruses from doing stupid shit like open email attachments from people they don't know or download files from shady sites.
Personally, AV software are a waste of time. I'm not going to spend half an hour scanning for crap when I can nuke it with a fresh reinstall. So far, life without AV's has been great.