I'll also be using Common Sense 2012.
Where'd you find that?! They quit selling that in 1948 :^D
It's also terribly misnamed because it just isn't that common anymore.
Well based on articles comparing free antivirus software, the top three would always be AVg, Avast and Avira. Since you already have used Avira and you are not contented with it, your next option will be aVast. You can also try AVG but it will also give you a lot of false alarms in a day.
I will recommend you to use licensed version of Norton if you wish to keep your system clean, but if you want a free one give a try to avg 2013 free.
Safe browsing habits are the best way to keep your computer clean.
What is this "sandbox" I keep hearing of? I have a built in Sandbox that I can run programs in, in Comodo, but this must be a virtual machine?