I say Microsoft Security Essentials 2.0 is best with Windows OS along each other no problem, light footprint.
Other free anti-virus are some good and some not effective.
Pay software is waste time, annoying pop-up pay fee after a year, etc...
If you like simple, light, quick, 64-bit, etc... that is Microsoft Security Essentials 2.0 make sense.
for small office or big business they do pay good brand for anti-virus for all around.
BTW, Best avoid Norton can give you lots of pain in azz, bog down on pc, etc..., check norton forum on google