Microsoft Security Essentials (MSE) is an antivirus software (AV) product that provides protection against different types of malware such as computer viruses, spyware, rootkits and Trojan horses. It runs on Windows XP, Windows Vista and Windows 7, but not on Windows 8, which has a built-in AV component. The license agreement allows home users and small businesses to install and use the product free of charge. It replaces Windows Live OneCare, a discontinued commercial subscription-based AV service, and the free Windows Defender, which until Windows 8 only protected users from adware and spyware.
Built upon the same virus definitions and scanning engine as other Microsoft antivirus products, MSE provides real-time protection, constantly monitoring activities on the computer and scanning new files as they are downloaded or created and disabling detected threats. It lacks the personal firewall of OneCare or the centralized management features of Forefront Endpoint Protection.
Microsoft's announcement of its own AV software on 18 November 2008 was met with mixed reactions from the AV industry. Symantec, McAfee and Kaspersky Lab—three competing independent software vendors—dismissed it as an unworthy competitor, but AVG Technologies and Avast Software appreciated its potential to expand the consumers' choice of AV software. AVG, McAfee, Sophos and Trend Micro claimed that the integration of the product into Microsoft Windows would be a violation of competition law.
The product received generally positive reviews praising its user interface, low resource usage and freeware license. It secured AV-TEST certification on October 2009, having demonstrated its ability to eliminate all widely encountered malware. However, it lost the certificate on October 2012, having shown a constant decline in protection and repair scores. In June 2013, MSE achieved the lowest possible protection score, zero. According to a March 2012 report by anti-malware specialist OPSWAT, MSE was the most popular AV product in North America and the second most popular in the world, which has resulted in the appearance of several rogue antivirus programs that try to impersonate it.