If you dont mind your computer slowing down, use avast.
If you want protection with performace, get avg. There is pratically no impact on performance with AVG, and I have tried both avast and AVG.
It all boils down the the real time protection. Avast is more intrusive, whilst AVG is more underlying and you dont notice its scanning EVERYTIME you open a new folder/window like you do with avast.
As for firewall, I use windows firewall. Its effective, and as long as you dont "make yourself a target" which means you dont go on "THOSE" websites, it should work well, and not cause programs to behave erratically or be blocked suddenly.
Add to this spybot, spyware blaster and ccleaner, and you are totally protected.
Havent had a single intrusion with this setup ever. No viruses or nothing.
Im kind of disappointed with this, isnce I used to like seeing how many things my virus scanner picked up.
Now everytime it scans, it reports 0. 0. 0.
So I just scan once a month as opposed to every week.