Despite all the negative reviews, Microsoft Security Essentials is still the most popular anti-virus worldwide with the market share of 15.3 percent, according to a report released by Opswat, MSE's market share was increased by 2.3 percent in December 2012.
Avast! Free Antivirus managed to get the second place with 13.6 percent, followed by Avira Free Antivirus with 9 percent and ESET Nod32 Antivirus with 6.8 percent.
Microsoft offers MSE via Windows Update as an optional update (KB KB2267621) to Windows XP, Windows Vista, and Windows 7 users. This might be the reason why MSE is so much popular in Windows users.