Microsoft Defender (known as Windows Defender before Windows 10 November 2019 Update or Windows Defender Antivirus in Windows 10 Creators Update and later) is an anti-malware component of Microsoft Windows. It was first released as a downloadable free anti-spyware program for Windows XP, and was later shipped with Windows Vista and Windows 7. It has evolved into a full antivirus program, replacing Microsoft Security Essentials as part of Windows 8 and later versions.
Before Windows 8, Windows Defender only protected users against spyware. It includes a number of real-time security agents that monitor several common areas of Windows for changes which might have been caused by spyware. It also has the ability to remove installed ActiveX software. Windows Defender featured an integrated support for Microsoft SpyNet 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 systems. Protection against viruses was subsequently added in Windows 8; which resembles Microsoft Security Essentials (MSE). It also uses the same anti-malware engine and virus definitions from MSE.
In Windows 10, Windows Defender settings are controlled in the Windows Defender Security Center. In the Windows 10 Anniversary Update, a new logo is introduced and a pop-up notification will appear to announce the results of a scan, even if no viruses are found.
Windows Defender was initially based on GIANT AntiSpyware, formerly developed by GIANT Company Software, Inc. The company's acquisition was announced by Microsoft on December 16, 2004. While the original GIANT AntiSpyware officially supported older Windows versions, support for the Windows 9x line of operating systems was later dropped by Microsoft.
The first beta release of Microsoft AntiSpyware from January 6, 2005 was a repackaged version of GIANT AntiSpyware There were more builds released in 2005, with the last Beta 1 refresh released on November 21, 2005.
At the 2005 RSA Security conference, Bill Gates, the Chief Software Architect and co-founder of Microsoft, announced that Windows Defender (formerly Microsoft AntiSpyware prior to November 4, 2005) would be made available free-of-charge to users with validly licensed Windows 2000, Windows XP, and Windows Server 2003 operating systems to secure their systems against the increasing malware threat.
Windows Defender (Beta 2) was released on February 13, 2006. It featured the program's new name and a redesigned user interface. The core engine was rewritten in C++, unlike the original GIANT-developed AntiSpyware, which was written in Visual Basic. This improved the application's performance. Also, since beta 2, the program works as a Windows service, unlike earlier releases, which enables the application to protect the system even when a user is not logged on. Beta 2 also requires Windows Genuine Advantage (WGA) validation. However, Windows Defender (Beta 2) did not contain some of the tools found in Microsoft AntiSpyware (Beta 1). Microsoft removed the System Inoculation, Secure Shredder and System Explorer tools found in MSAS (Beta 1) as well as the Tracks Eraser tool, which allowed users to easily delete many different types of temporary files related to Internet Explorer 6, including HTTP cookies, web cache, and Windows Media Player playback history. German and Japanese versions of Windows Defender (Beta 2) were later released by Microsoft.
On October 24, 2006, Microsoft released Windows Defender. It supports Windows XP and Windows Server 2003; however, unlike the betas, it does not run on Windows 2000.
Windows Defender was released with Windows Vista and Windows 7, serving as their built-in anti-spyware component. In Windows Vista and Windows 7, Windows Defender was superseded by Microsoft Security Essentials, an antivirus product from Microsoft which provided protection against a wider range of malware. Upon installation, Microsoft Security Essentials disabled and replaced Windows Defender. In Windows 8, Microsoft upgraded Windows Defender into an antivirus program very similar to Microsoft Security Essentials for Windows 7 and using the same virus definition updates. MSE itself does not run on Windows versions beyond 7. In Windows 8 and Windows 10, Windows Defender is on by default. It switches itself off upon installation of a third-party anti-virus package.
Starting with Windows 10, Microsoft began to transfer the control of Windows Defender out of its native client. Initially, its "Settings" dialog box was replaced by a dedicated page in the Settings app. In Windows 10 Creators Update, Windows Defender is renamed Windows Defender Antivirus to distinguish it from Windows Defender Security Center. The latter has become the default avenue to interface with Windows Defender. While there is no shortcut on the Start menu for Windows Defender's native client, it can still run. It was later removed in the Windows 10 April 2018 Update and transferred to Windows Defender Security Center.
Windows 10's Anniversary Update introduced Limited Periodic Scanning, which optionally allows Windows Defender to scan a system periodically if another antivirus app is installed. It also introduced Block at First Sight, which uses machine learning to predict whether a file is malicious.
Windows Defender had additional functionality in Windows Vista which was removed in subsequent versions of Windows:
Security agents which monitor the computer for malicious activities:
The Advanced Tools section allows users to discover potential vulnerabilities with a series of Software Explorers. They provide views of startup programs, currently running software, network connected applications, and Winsock providers (Winsock LSPs). In each Explorer, every element is rated as either "Known", "Unknown" or "Potentially Unwanted". The first and last categories carry a link to learn more about the particular item, and the second category invites users to submit the program to Microsoft SpyNet for analysis by community members. The Software Explorer feature has been removed from Windows Defender in Windows 7.
Windows Defender in Windows Vista automatically blocks all startup items that require administrator privileges to run (this is considered suspicious behavior for a startup item). This automatic blocking is related to the User Account Control functionality in Windows Vista, and requires users to manually run each of these startup items each time they log in if they desire the item to run at startup.
In Windows Vista, it is possible to close the window and have the program run in the system tray while a scan is running. However, in Windows 7, this functionality was removed and the window must remain open while a scan is running.
Windows Defender Offline (formerly known as Standalone System Sweeper) is a bootable standalone anti-malware program that runs from a bootable disk designed to scan infected systems while their operating systems are offline. Since Windows 10 Anniversary Update, the offline functionality is integrated into the regular Windows Defender program as well.
During the December 2017 test of various anti-malware software carried out by AV-TEST on Windows 10 platform, Windows Defender has earned 6 out of 6 points in detection rate of various malware samples, earning its "AV-TEST Certified" seal. Also, during February 2018 "Real-World Protection Test" performed by AV-Comparatives, Windows Defender has achieved 100% detection rate of malicious URL samples, along with 3 false positive results.
AV-TEST test of Defender October 2019 shows it provides excellent protection both against viruses and 0-day / malware attacks.