Information taken from: http://www.microsoft.com/security_essentials/
About Microsoft Security Essentials
Microsoft Security Essentials provides real-time protection for your home PC that guards against viruses, spyware, and other malicious software.
Microsoft Security Essentials is a free* download from Microsoft that is simple to install, easy to use, and always kept up to date so you can be assured your PC is protected by the latest technology. It’s easy to tell if your PC is secure — when you’re green, you’re good. It’s that simple.
Microsoft Security Essentials runs quietly and efficiently in the background so that you are free to use your Windows-based PC the way you want—without interruptions or long computer wait times.
*Your PC must run genuine Windows to install Microsoft Security Essentials.
Other Anti-Virus Software
If you'd prefer not to use Microsoft Security Essentials, you may also choose from one of the following free anti-virus solutions: