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.

According to OPSWAT, the free version of Avira Antivir Personal comes in second place as most used antivirus worldwide with a share of 10.18%, while AVAST! Free Antivirus is third with 8.66%.
to read the full article.

(view the complete list along with the usage percentage).