I am using Avira - I have tried most of the ones out there, and they each have things I just don't like about them - but the paid ones were just as bad. Norton really screwed up a laptop I was using it on, and McAfee was worthless.
I am using Avast, but is good to pay attention what files you are installing because no antivirus offers 100% protection.
as per my concern norton anti virus is good enough. For free ware i used avast and now using microsoft essential but it needs original windows.
I am using at the moment a couple of antiviruses:Avast,McAfee,Avira and Kaspersky,it's very good ,however for the smallest thing it alerts me.
I used to go through alot of those free ones, trying out different ones. But honestly, I stopped after I downloaded and installed AVG. To this day, thats what I use. Paid and free, they are both good, very good. AVG isn't so bloated either so thats what makes it that much better