Windows Defender is Microsoft's antimalware software. Defender works with Windows Update to automatically install new virus definition files as they are published and provide computers using the Microsoft operating system (OS) with both real-time and anytime scanning options.
Defender was originally released in 2006 as an add-on for Windows XP. With the release of Windows Vista, however, the software was enabled by default. Since the release of Windows 8, should a computer's default antivirus software be from a third-party and that software expire, Defender will automatically become the default AV once again. This is to ensure that every computer running a Microsoft operating system is using antivirus software.
Windows Defender is considered to be primarily a consumer-oriented security tool, geared toward the non-technical user who is not interested in purchasing additional protection. It does have the advantage of knowing the Windows OS better than any other tool and is quite good at removing malware when it is detected. Because it is a Microsoft product, however, it is also a target for malware designed specifically to attack and disable Defender.