I work at a computer repair shop. I would say Norton antvirus is number 1 however it costs like 40 bucks. The next best which is free is "Microsoft Security Essentials". Just look it up and it should do the trick.
I'm not saying this because I helped with the project but the very best
Anti-Virus Software for a Windoze machine is written by Computer Associates, the number one software company in the world and is found at ca.com. The only ads you see for it are by retailers that are running a sale on it, it's that good! Next to that is Malware Bytes. I no longer will run Windoze except to run one of my Remote Control Airplane Sims and run the, infinitely, safer Ubuntu/Linux. For as long as I ran Windoze, I ran the CA program with zero infestations.