I don't use any forms of anti virus or security system since i was 10, at least i can tell what's danger and what's not.
It's true that everybody will at least got virus once and that including me, i got this sassers when i was readying my hundred and eight pc for my cyber cafe opening using windows xp home, that time sp2 wasn't available yet and guess what, i spent 3 days without sleep to finish setup the whole server and client pc, and one xxxxing mistake i made that i accidentally click on a stupid pop ups (i can't even remember my name that time, due to not sleeping for days), and it screws my days of hard work with a message says your pc will shut down in 60 secs, that's one of the worst day in my life i bet..
and another time that's funny is like recently in 2008. I'm using this Motorola V3xx and i don't use to go internet with my phone, that day i went internet with my phone and do nothing,(just to see what my phone can do), and my phone actually caught with virus and i don't know, then when i double click the memory card after i plug my phone in to transfer some files, my pc freeze for like 1~2seconds (usually it won't), and i say to myself, i'm screwed(i'm not using any antivirus and security thingy, it slows my pc a lot which i hate), and i'm forced to reformat my HDD lol and took me 6 hours just to reinstall and make it just like how it was before i got that virus...lol
Yea over the 12 years of my life with computer I do get virus from time to time, yet i don't use any sort of security software, I do believe that if somebody want to crack ur pc open, they can and they will and they do it and they succeed anyhow anyway