Windows Update is a service provided by Microsoft for users of Microsoft Windows which provides updates for the operating system and its installed components. Microsoft Update is an optional feature that can be enabled to provide updates for other Microsoft software installed on a Windows computer, such as Microsoft Office, Windows Live applications, and Microsoft Expression.

Security updates are delivered by the Windows Update service on the second Tuesday of each month (Patch Tuesday). However, security updates can be delivered whenever a software update is required to prevent an exploit targeting Windows users. Windows Update can be configured to automatically install updates, ensuring that a computer is always up-to-date and not vulnerable to computer worms and other such malware.

Windows Vista and Windows Server 2008 provide a Control Panel to configure update settings and check for updates. The Windows Update Control Panel is also the means to download Windows Ultimate Extras (optional software offered to users of Windows Vista Ultimate Edition). For previous versions of Microsoft Windows, updates can be downloaded from the Windows Update website, using Internet Explorer.

Windows Update web site

Windows Update requires Internet Explorer or a third-party web browser that uses Microsoft's MSHTML layout engine. This necessity stems from the use of an ActiveX control to house the software that is executed on the user's computer. While the underlying implementation details have changed from version to version, its basic purpose of scanning the user's computer to see what operating system components and software is installed, and comparing the versions of those components with what is offered by Windows Update. The ActiveX component then interfaces with Windows Installer in order to install or update those components, and to report the success or failure of those installations back to Microsoft's servers.

The first version of the Windows Update web site (usually referred to as "v3") did not require any personally identifiable information to be sent to Microsoft. In order for the v3 ActiveX control to determine what updates were needed, the entire list of available software on Windows Update was downloaded to the user's computer when they visited the Windows Update web site. As the number of updates offered by Windows Update grew, this resulted in performance concerns. Arie Slob, writing for the Windows-help.net newsletter in March 2003 noted that the size of the update list had exceeded 400KB, which caused delays of more than a minute for dial-up users.

Windows Update v4, released in conjunction with Windows XP in 2001, changed this by having the ActiveX control submit a list of the hardware components to Microsoft's servers, which then returns a shortened list of device drivers that are available for that machine. It also narrowed down the list of available updates for the operating system and related components by sending details of what operating system version, service pack, and locale are installed. German technology web site tecchannel.de published an analysis of the Windows Update communication protocol in February 2003, which received wide attention on technology web sites. The report, which was the first to contain extensive details of how the Windows Update communication protocol worked, also discovered that the make and model of the computer, the amount of free disk space, and the Windows product key.