While details have changed from version to version, it has always scanned the computer to find what operating system components and software are installed, and compared the versions of those components with the latest available versions.
The Active X component then executes the downloaded installation files in their silent/unattended mode to install or update those components, and to report the success or failure of those installations back to Microsoft's servers.
Windows Update is a service provided by Microsoft for users of Microsoft Windows which installs software updates for the operating system and its installed components.
Microsoft Update, an expanded version of the service, provides updates for other Microsoft software as well, such as Microsoft Office.
Microsoft routinely releases security updates on the second Tuesday of each month (Patch Tuesday), but can provide them whenever a new update is urgently required to prevent a newly discovered or prevalent exploit targeting Windows users.
With the release of Windows ME in 2000, Microsoft introduced Automatic Updates as a replacement for the Critical Update Notification tool.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 Active X 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.Windows Update clients normally download updates over an Internet connection, although Microsoft makes provision for installing updates on computers without an Internet connection and a range of alternative methods and programs also exist to install updates. Security updates or critical updates protect against vulnerabilities to malware and security exploits.Other updates correct errors unrelated to security, or enhance functionality.
Shortly after the release of Windows 98, Microsoft released a Critical Update Notification Tool (later called Critical Update Notification Utility) through Windows Update, which installed a background tool on the user's computer that checked the Windows Update web site on a regular schedule for new updates that have been marked as "Critical".