“Microsoft Security Essentials is a free antivirus program for your personal computer, and provides real-time protection against viruses, spyware and other malicious software.
The program runs in the background while you're using your PC and checks daily for any updates from the Internet. It performs scheduled scans of your computer for any malware, automatically blocking most threats. Basically, if your Microsoft Security Essentials icon is displayed as green in the notification area, at the far right of the taskbar, your computer is in good condition. If the program detects a problem or threat, it displays a message with the detected item, the alert level, and the recommended action, so you can decide how to proceed. You can also use Microsoft Security Essentials to perform a manual scan of a file, a folder, or your entire PC, at any time. ”
Read more about it, including system requirements and how to install at What is Microsoft Security Essentials?
For more information, go to the Microsoft Security Essentials website