The previous posters are partly correct. a smart user is the best defense, BUT there are some fantastic security tools that help a great deal. Unfortunately, windows defender is not one of them.
Real security doesn't start with anti-virus. You want to keep stuff off your system to begin with. To that end, intrusion control software is the best approach. The best in the business in my opinion is Outpost firewall, or Comodo. Both come as stand alone intrusion control/firewall or in full security suites with antivirus. But require a fair amount of user input, which is their strength and weakness. They are only as good as the user that controls them. If you know what you are doing, they are VERY effective.