Here's how I see the popular free Antiviruses today,
Comodo Internet Security = Best array of features for free. HIPS, Sandbox, Firewall + endless setting options.
Qihoo = Very EFFECTIVE with multiple engines, provides top protection. Can even fix an already infected PC.
Avast = Good overall in all areas and even better with "Hardened Mode" activated.
Avira = Top signatures updated ever 2 hours. Lightest feel overall, no drag on system, even on old hardware.
AVG = Getting a lot stronger and catching many PUP's and threats of all kinds.
Bitdefender = Simple, light, minimal, boot time protection better than paid AV and top notch signatures.
Panda = Soild protection, light cloud, easy on drive and easy to use.
I could use any of these Antiviruses and enjoy their functions and added security.
.