I use Avast Free Edition, never had ANY problems with it at all, and haven't had a virus since I started using it. Kaspersky is the number 1 rated Anti-Virus on the market though.
I have been using AVG freee (now V8) for the last coupple of years and no problems. I would not go back to Norton, which I previously used. I think it is a big mess!
I've got Norton at the moment & not real happy with it. Might try Kaspersky or Nod32 next ...