I've had good luck with avast free for the last few years. The auto sandbox can be a bit aggressive at times so set it to ask instead of auto, or disable sandboxing.
MSE (Microsoft security essentials) is one of the easiest to set up and integrates well with windows. Others have had good success with it, but I stopped using it a year ago when three of our computers at work got infected while using MSE. I would have given it another chance, but it was three different viruses that infected these machines. Anyhow, I'm sure improvements have been made in the past year.