Initial releases of Windows 2000 shipped with the tool, but Windows 95 and Windows NT 4.0 were not supported.
It was superseded by Automatic Updates in Windows Me and Windows 2000 SP3.
Windows Update requires Internet Explorer or a third-party web browser that uses Microsoft's MSHTML layout engine, as it must support the use of an Active X control to house the software that is executed on the user's computer.
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.
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.
German technology web site published an analysis of the Windows Update communication protocol in February 2003, which received wide attention on technology web sites.If that one server got compromised one day, or an attacker cracks the MS DNS server again, there could be millions of users installing trojans every hour.The scope of this attack is big enough to attract crackers who actually know what they are doing..." The Critical Update Notification tool continued to be promoted by Microsoft through 1999 and the first half of 2000.Arie Slob, writing for the 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 Active X control submit a list of the hardware components to Microsoft's servers, which then returns a list of only those device drivers available for that machine.