No matter what type of computer you are using, we recommend you install anti-virus software on it. You can find information on our recommendations here. If you have anti-virus software installed on your computer, the next step is keeping your system up to date.
Mac users can use the App Store in Mountain Lion and later to keep their computers up to date. More information on that can be found HERE on the Apple Website.
You can check to see if you have the latest critical patches for your Windows-based PC at http://windows.microsoft.com/en-us/windows/windows-update.