Go with Bitdefnder. Top ranking over Avg, Avast, Kapersky, etc in the last few major tests run by respected 3rd party reviewers. They have a free version as well as paid.
Also pick up Malwarebytes. The free version doesn't run resident, but it's great for on demand scanning.
If you can get your hands on a copy of ESET, its the best imo. I've had two customers that call me every 2 weeks with software issues and I used avg and bitdefender and neither programs stopped the problem.
I tried ESET, recommended by the microcenter rep and neither customer has had an issue they couldn't resolve on their own.
I do LOVE the interface with avast free but I have slight hesitance with their software since they got busted for imbedding software in their antivirus program to collect customer data for their own gain.
I second malwarebytes, free is fine. Just do the manual scan at least once a week.