Since the release of Windows 1.0 in November 1985, Microsoft has produced many versions of Windows operating systems with improvements and changes to support a variety of users and purposes. Figure summarizes the current Windows OS.

NT 4 was designed to provide an environment for mission critical business that would be more stable than the Microsoft consumer operating systems. It is available for both desktop (NT 4.0 Workstation) and server (NT 4.0 Server). An advantage of NT over previous Microsoft OSs is that DOS and older Windows programs can be executed in virtual machines (VMs). Program failures are isolated and do not require a system restart. 

Windows NT provides a domain structure to control user and client access to server resources. It is administered through the User Manager for Domains application on the domain controller. Each NT domain requires a single primary domain controller which holds the Security Accounts Management Database (SAM) and may have one or more backup domain controllers, each of which contains a read-only copy of the SAM. When a user attempts to logon, the account information is sent to the SAM database. If the information for that account is stored in the database, the user will be authenticated to the domain and have access to the workstation and network resources.

Based on the NT kernel, the more recent Windows 2000 has both desktop and server versions. Windows 2000 supports “plug-and-play” technology, permitting installation of new devices without the need to restart the system. Windows 2000 also includes a file encryption system for securing data on the hard disk.

Windows 2000 enables objects, such as users and resources, to be placed into container objects called organizational units (OUs). Administrative authority over each OU can be delegated to a user or group. This feature allows more specific control than is possible with Windows NT 4.0.

Windows 2000 Professional is not designed to be a full NOS. It does not provide a domain controller, DNS server, DHCP server, or render any of the services that can be deployed with Windows 2000 Server. The primary purpose of Windows 2000 Professional is to be part of a domain as a client-side operating system. The type of hardware that can be installed on the system is limited. Windows 2000 Professional can provide limited server capabilities for small networks and peer-to-peer networks. It can be a file server, a print server, an FTP server, and a web server, but will only support up to ten simultaneous connections.

Windows 2000 Server adds to the features of Windows 2000 Professional many new server-specific functions. It can also operate as a file, print, web and application server. The Active Directory Services feature of Windows 2000 Server serves as the centralized point of management of users, groups, security services, and network resources. It includes the multipurpose capabilities required for workgroups and branch offices as well as for departmental deployments of file and print servers, application servers, web servers, and communication servers.

Windows 2000 Server is intended for use in small-to-medium sized enterprise environments. It provides integrated connectivity with Novell NetWare, UNIX, and AppleTalk systems. It can also be configured as a communications server to provide dialup networking services for mobile users. Windows 2000 Advanced Server provides the additional hardware and software support needed for enterprise and extremely large networks.

Windows .NET Server is built on the Windows 2000 Server kernel, but tailored to provide a secure and reliable system to run enterprise-level web and FTP sites in order to compete with the Linux and UNIX server operating systems. The Windows .NET Server provides XML Web Services to companies which run medium to high volume web traffic.