Microsoft Security Essentials (MSE) is a free antivirus software created by Microsoft that provides protection against viruses, spyware, rootkits, and trojans for Windows XP, Windows Vista, and Windows 7. It is completely free, as long as you are running a legal copy of windows.
Security Essentials which has such a clean, minimalist interface. There are only four tabs at the top to navigate through (Home, Update, History, and Settings) and none of the pages are cluttered with content. Updates and upgrades are automatic, so there’s no need to worry about having the latest protection. It’s easy to tell if you’re protected – when the Security Essentials icon is green, your status is good.
Download Security Essentials from http://www.microsoft.com/security_essentials