Microsoft Security Essentials is the most popular security solution worldwide according to a report by OPSWAT covering the second quarter of 2011, March 23 to May 15, 2011.
Microsoft’s free antivirus offered to users of genuine Windows operating systems including Windows 7 RTM and SP1, has been found to run on no less than 10.66% of the 43,000 computers around the world covered by the OPSWAT’s Q2 Antivirus Market Share Report.
Microsoft Security Essentials, now at version 2.0 which was released at the end of 2010, is a free, basic security solution that the software giant launched initially in 2009. The Redmond company’s only statistics about MSE made public reveal that the AV has long passed the 30 million downloads mark.
I use and recommend MSE a lot these days. It has become one of the first things I'll put on a new install of Windows.