What is the System Registry?
The Windows Registry is a set of data files used to help Windows control hardware, software, the user's environment, and the "look and feel" of the Windows interface. The Registry is contained in two files in the Windows directory (in XP they are called "hives"): system.dat and user.dat, with backup copies system.da0 and user.da0. The Registry database is accessed with regedit.exe which is in the Windows directory. Formerly, in older version of windows (before windows 95) these functions were performed by WIN.INI, SYSTEM.INI, and other .INI files that are associated with applications.
Originally, SYSTEM.INI and WIN.INI controlled all Windows and application features and access. It worked well when average users used only a few applications. As the number and complexity of applications grew, so did the number of entries to the .INI files. The downside of this approach, in a growing environment, is that everyone would make changes to the .INI files when applications were added to the system. However, no one ever removed references from their .INI files when they removed applications, so SYSTEM.INI and WIN.INI continued to get larger and larger. Each incremental size increase meant slower performance. Even upgrading applications presented its challenges. The upgrade would add entries but never take the old ones away, presumably to ensure compatibility if another program was to access the settings.
The Registry is a set of files that control all aspects of the operating system and how it works with outside events. Those "events" range from accessing a hardware device directly to how the interface will react to a specific user to how an application will be run and much more. It was designed to work exclusively with 32-bit applications, and file size is limited to about 40MB.
The Registry is complex by its very nature, and on purpose.
It is the data file for all 32-bit hardware/driver combinations and 32-bit applications in both Windows XP and Windows 95. Sixteen-bit drivers do not work in NT, so all devices are controlled through the Registry, even those normally controlled by the BIOS. In Windows 95, 16-bit drivers will continue to work as real-mode devices, and they use SYSTEM.INI for control.
Without the Registry, the operating system would not have the necessary information to run, to control attached devices, to launch and control applications, and to respond correctly to user input.
When a user attempts to launch an application, the Registry supplies application information to the OS so the application can be found, the correct data file locations are set, and other settings are available.
The Registry holds information about default data and auxiliary file locations, menus, button bars, window status, and other options. It also holds installation information such as the date of installation, the user who installed the software, the version number and date, and sometimes the serial number. Depending on the actual software installed, it may contain other application-specific information.
Without the Registry, Windows 95 and Windows XP would not be possible. They are too complex to be controlled by the older .INI files, and their expansion capabilities allow almost unlimited installation and use of applications. The Registry is, however, much more complex than the .INI files, and understanding how it works, what it does, and how to work with it is critical for effective system administration.
The Registry controls all 32-bit applications and their functions on the system, plus the interaction between multiple applications, such as copying and pasting. It also controls all the hardware and drivers. Though most of the settings are made during installation and through the Control Panel, understanding the Registry is fundamental to reliable and capable management of Windows XP and Windows 95 systems.