Avast is my favorite as this one is the best free antivirus voted by consumers. So I installed it & happy with this one.
I haven't used an anti virus since Windows ME, I use Firefox and I have never had a virus on any of my computers since.
Most of the time I use a Mac so there's no need for an anti-virus. But last time I used a PC I think I used Avaira as it was recommended.