Avira Antivir
http://www.free-av.com/
AVG9
http://free.avg.com/us-en/download?prd=afg
Avira has a pop up on updates, but take out less ressources and to my experience screws about less with your random stuff (Scalix outlook connector or other things)
AVG9 has no popup on update to my experience (dont know if it changed, havent used it in a bit) Has its own ads within the antivirus consol (less annoying) but takes a bit more ressources.
They both destroy Norton and McCoffee Garbage sold by futureshop for retarded prices.