Microsoft Security Essentials（简称MSE），是一款由微软开发及发行的免费杀毒软件，用于防止计算机病毒、间谍软件、恶意软件、Rootkit和木马，可以运行于Windows XP、Windows Vista、Windows 7上，但无法在Windows 8上运行（Windows 8已经内置Windows Defender）。其代号为Morro，意为“小丘”。MSE的最终用户许可协议允许家庭用户和小企业免费安装和使用本产品。它取代了Windows Live OneCare和Windows 7以前的Windows Defender。与市面上其他免费防恶意软件均有提供更全面的保护（例如防火墙功能）的收费版本不同，MSE只有免费版本。

Microsoft Security Essentials (MSE) is an antivirus software (AV) product that provides protection against different types of malicious software such as computer viruses, spyware, rootkits and Trojan horses. Before version 4.5, MSE ran on Windows XP, Windows Vista and Windows 7, but not on Windows 8 and later, which have built-in AV components. MSE 4.5 and later do not run on Windows XP. 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 protected users from only adware and spyware.