The first beta release of Microsoft AntiSpyware from January 6, 2005, was a repackaged version of GIANT AntiSpyware. 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 company's acquisition was announced by Microsoft on December 16, 2004.

Windows Defender was initially based on GIANT AntiSpyware, formerly developed by GIANT Company Software, Inc. Microsoft AntiSpyware Beta 1 (Version 1.0.701) running on Windows XP 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. In Windows 10, Windows Defender settings are controlled in the Windows Defender Security Center. It also uses the same anti-malware engine and virus definitions from MSE. Protection against viruses was subsequently added in Windows 8 which resembles Microsoft Security Essentials (MSE).

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. It also has the ability to remove installed ActiveX software. 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.

4.3 Notification of startup programs that run as an administratorīefore Windows 8, Windows Defender only protected users against spyware.

The mobile version of Microsoft Defender also includes a feature to block access to corporate data if it detects a malicious app is installed. On March 21, 2019, Microsoft announced Microsoft Defender ATP for Mac for business customers to protect their Mac devices from attacks on a corporate network, and a year later, to expand protection for mobile devices, announced Microsoft Defender ATP for Android and iOS devices, which incorporates Microsoft SmartScreen, a firewall and malware scanning. Compared to other antivirus, Microsoft Defender lacks advanced security features such as a password protector, sandbox, or website phishing checker, yet it offers protection against malware installed on the computer. It has evolved into a full antivirus program, replacing Microsoft Security Essentials, as part of Windows 8 and later versions. It was first released as a downloadable free anti-spyware program for Windows XP, and was later shipped with Windows Vista and Windows 7. Windows Security (known as Windows Defender Antivirus before Windows Update, Windows Defender before Windows 10 Creators Update, or Microsoft Defender Antivirus before Windows 11) is an anti-malware component of Microsoft Windows.

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