Run Microsoft Security Essentials. Turn on the default Windows Firewall on your machine. Use the security settings in your browser. Get the freeware AVG A/V program at
http://www.avg.com
As pointed out, while no system is 100% safe, if you really want to make yourself less vulnerable and are not dependent on too many Windows applications, you can download the free CentOS or Ubuntu Linux OS's. While they are not 100% clean, there are significantly fewer viruses that have been adapted for those operating systems. And the latest Ubuntu is very Mac OSX-like.