 Microsoft Security Essentials is an antivirus software ( AV ) product that fights malicious software ( malware ) , including computer viruses , spyware , Trojan horses and rootkits . 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 . It automatically checks for and downloads the virus definitions it relies on from Microsoft Update , a web @-@ based software service updated three times a day . Users may alternatively download the updates manually from the Microsoft Security Portal website . On 30 September 2011 , a faulty definition update caused the product to incorrectly tag Google Chrome as malware . The issue was resolved within three hours . MSE originally ran on Windows XP , Windows Vista and Windows 7 , although versions 4 @.@ 5 and later do not run on Windows XP and Microsoft stopped producing definition updates for Windows XP on 14 July 2015 . 
