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Windows Defender
Windows Defender, formerly known as Microsoft AntiSpyware, is a software product that helps combat malware. Windows Defender was initially an antispyware program; it is included with Windows Vista and Windows 7 and is available as a free download for Windows XP and Windows Server 2003. In Windows 8, however, it is upgraded to an antivirus program.
Continued: http://en.wikipedia.org/wiki/Windows_Defender