I have tried, among others, Norton, McAfee, Panda, Avast, AVG, PCillin, Kaspersky (KAV), BitDefender and NOD32 because after suffering 6 severe disasters due to failures in McAfee (3) and Norton (3) I became kind of anti-virus obsessed.
Since theres is no free meal, a free anti-virus is, for sure, lacking something. For me the 3 best are BitDefender 9 Std, KAV 5 Personal and NOD32.
BitDefender freezes my PC for anout 15 seconds whenever it updates its virus data bases, but it allows to keep detailed record of every infected object found and has by far the best technical support. Out of the three, it's the second easier to customize.
KAV Personal 5 is too heavy, uses too many PC resources, put "garbage" (KAVICH$:) into my PC --and, even though, takes a long to scan-- and is almost "mute", which is bad for me because I want to keep record of every infected/suspicious object being intercepted. It's the easiest to customize.
NOD32 has the highest virus detection rate, is made in a way that resembles no other one I know of, is no as "mute" as KAV, uses almost no PC resources and fully scan my PC in 1 hour while BitDefender takes 1,5 and KAV 2.5. It's the more difficult to customize. So I have stayed with NOD32 ... till I discover something better.