"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." The Redmond giant said that it is not out to steal business from anti-virus companies such as McAfee or Symantec Corporation.
Instead, Microsoft is hoping that people who do not already run anti-virus software, and will not consider buying anti-virus software, will be the ones who choose to run Microsoft Security Essentials.
More info: http://www.microsoft.com/security_essentials/