I use Firefox and McAfee Kitty.

I renewed mine when the trial period expired, and although I've tried other anti-virus programs, this one suits me the best.

I did use AVG for a while but it let through so many Trojan Horses and so much spyware, that I changed to Avast which is another free AV program.

http://www.avast.com/eng/download-avast-home.html

Even that wasn't perfect, (I still got the occassional Trojan) but it was better than AVG.