Turn on the Firewall and Disable File & Printer Sharing. These are two of the most effective ways of blocking anyone from hacking your machine wherever your use your computer. Below you find links to how to perform these steps for the appropriate Operating Systems.
Here are some firewalls that will help improve security. There are two options for Windows and two for MAC. You only need one. Read the features and search reviews before you use them. I have used ZoneAlarm in the past but with the functions I need to perform it hinders my abilities. Windows – ZoneAlarm or Comodo — MAC – WaterRoof or NoobProof
Here are some free downloads from Microsoft to help protect your system. I have used the first two and the add very little overhead to your machine. Microsoft MSRT / Windows Defender / Security Essentials
This is a great tool for scanning you Windows machine for vulnerabilities and needed updates. Microsoft Baseline Security Analyzer