That is my question.
I've never run any a/v software on my XP system. No need.
I don't use any Microsoft email software. I don't open shady attachments. I exclusively use Firefox or Safari (that is to say, not Internet Explorer). Even with FF I always use Adblock to protect myself from even more things. I don't run a software firewall but I am behind a router - so that will block most rogue/random crap out there.
Then there's the actual effectiveness of antivirus apps. We use Symantec products at work and I have to say, not impressed. We have a ridiculous number of users who still manage to get infected with SOMETHING even though we have firewalls, a/v software, security software, etc, etc. So why should I run something that in the ridiculously off chance that I am "attacked" by something probably wouldn't help me anyway?
Don't forget running any sort of real time protection takes away from your system resources. Not that my system is maxed out and I need to save power or anything... I just want to know that everything is available if I want it. I'd be better off just running an online scan if there's a problem. Or booting to a live CD or PE environment.
Now if anything, I'll keep an anti spyware app handy (MalwareBytes FTW). But I likely wouldn't even install it unless I suspected something. And the chances of that are so ridiculously low... well, like I said, I'll run it if I think something is wrong.
I suppose I just worked out my decision. No antivirus. Call me crazy or foolish... or stupid; but I'm sure you'll be the first to know if I find a problem - but I wouldn't advise waiting by the phone or anything.