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Windows Update is a Microsoft service for the Windows 9x and Windows NT families of operating system, which automates downloading and installing Microsoft Windows software updates over the Internet. The service delivers software updates for Windows, as well as the various Microsoft antivirus products, including Windows Defender and Microsoft Security Essentials. Since its inception, Microsoft has introduced two extensions of the service: Microsoft Update and Windows Update for Business. The former expands the core service to include other Microsoft products, such as Microsoft Office and Microsoft Expression Studio. The latter is available to business editions of Windows 10 and permits postponing updates or receiving updates only after they have undergone rigorous testing.
As the service has evolved over the years, so have its client software. For a decade, the primary client component of the service was the Windows Update web app that could only be run inside Internet Explorer. Starting with Windows Vista, the primary client component became Windows Update Agent, an integral component of the operating system.
The service provides several kinds of updates. Security updates or critical updates mitigate vulnerabilities against security exploits against Microsoft Windows. Cumulative updates are updates that bundle multiple updates, both new and previously released updates. Cumulative updates were introduced with Windows 10 and have been backported to Windows 7 and Windows 8.1.
Microsoft routinely releases updates on the second Tuesday of each month (known as the Patch Tuesday), but can provide them whenever a new update is urgently required to prevent a newly discovered or prevalent exploit. System administrators can configure Windows Update to install critical updates for Microsoft Windows automatically, so long as the computer has an Internet connection.

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