Windows Defender

Windows Defender, formerly known as Microsoft AntiSpyware, is a software product from Microsoft to prevent, remove and quarantine spyware in Microsoft Windows. It is included and enabled by default in Windows Vista and Windows 7, and is available as a free download for Windows XP and Windows Server 2003.
Basic features
Windows Defender features system scan capabilities similar to other free products on the market,[clarification needed] and includes a number of real-time security agents that monitor several common areas of Windows for changes which may be caused by spyware. It also includes the ability to easily remove ActiveX applications that are installed. Also integrated is support for Microsoft's SpyNet network that allows users to report to Microsoft what they consider to be spyware, and what applications and device drivers they allow to be installed on their system.

Final release
On October 24, 2006, Microsoft released the final version.[5] It supports Windows XP, Windows Server 2003 and Windows Vista; however, unlike the betas, it does not run on Windows 2000 by default.