Avast is a good free antivirus and Comodo is a good free firewall.
But the thing I most highly suggest is using the noscript add-on for firefox.
I use all three.
I've done quite a bit of experimenting lately and it seems different ones work better on different computers in my house. If I had to choose I would go with the flo here and say Avast
I've been using AVG for years but you have to be careful and disable the link scanner or your Internet will drag. The link scanner scans links before it lets your browser go to the web site.
AVG already has a Resident Shield module which scans files in the background. They somewhat duplicate each other's activities so I just disable the link scanner and viruses from web sites are caught by the Resident Shield.