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Windows Registry Explained

The Microsoft Computer Dictionary, Fifth Edition, defines the registry as:

A central hierarchical database used in Microsoft Windows 98, Windows CE, Windows NT, and Windows 2000 used to store information that is necessary to configure the system for one or more users, applications and hardware devices.

The Registry contains information that Windows continually references during operation, such as profiles for each user, the applications installed on the computer and the types of documents that each can create, property sheet settings for folders and application icons, what hardware exists on the system, and the ports that are being used.

The Registry replaces most of the text-based .ini files that are used in Windows 3.x and MS-DOS configuration files, such as the Autoexec.bat and Config.sys. Although the Registry is common to several Windows operating systems, there are some differences among them.

Structure

A registry hive is a group of keys, subkeys, and values in the registry that has a set of supporting files that contain backups of its data. The supporting files for all hives except HKEY_CURRENT_USER are in the %SystemRoot%\System32\Config folder on Windows NT 4.0, Windows 2000, Windows XP, Windows Server 2003, and Windows Vista. The supporting files for HKEY_CURRENT_USER are in the %SystemRoot%\Profiles\Username folder. The file name extensions of the files in these folders indicate the type of data that they contain. Also, the lack of an extension may sometimes indicate the type of data that they contain.

HKEY_CLASSES_ROOT
Abbreviated HKCR, HKEY_CLASSES_ROOT stores information about registered applications, including associations from file extensions and OLE object class ids to the applications used to handle these items. On Windows 2000 and above, HKCR is a compilation of HKCU\Software\Classes and HKLM\Software\Classes. If a given value exists in both of the subkeys above, the one in HKCU\Software\Classes is used.

HKEY_CURRENT_USER
Abbreviated HKCU, HKEY_CURRENT_USER stores settings that are specific to the currently logged-in user. The HKCU key is a link to the subkey of HKEY_USERS that corresponds to the user; the same information is reflected in both locations.

HKEY_LOCAL_MACHINE
Abbreviated HKLM, HKEY_LOCAL_MACHINE stores settings that are general to all users on the computer. This key is found within the file %SystemRoot%\System32\Config\system on NT-based versions of Windows. Information about system hardware is located under the SYSTEM key.

HKEY_USERS
Abbreviated HKU, HKEY_USERS contains subkeys corresponding to the HKEY_CURRENT_USER keys for each user registered on the machine.

HKEY_CURRENT_CONFIG
Abbreviated HKCC, HKEY_CURRENT_CONFIG contains information gathered at runtime; information stored in this key is not permanently stored on disk, but rather regenerated at boot time.

HKEY_PERFORMANCE_DATA
This key provides runtime information into performance data provided by either the NT kernel itself or other programs that provide performance data. This key is not displayed in the Registry Editor, but it is visible through the registry functions in the Windows API.

Editing The Registry

It is generally not recommended that you edit the registry unless you know what you are doing. Changes to the registry can result in serious operating system problems and/or failure. I learned this from experience, as my first endeavours into the registry led to a complete Windows failure.

Make sure you backup your registry before performing any edits!!

Manual Editing: The registry can be edited manually in Microsoft Windows by running regedit.exe or regedt32.exe in the Windows directory. You can quickly get to regedit by clicking Start -> Run, then type "regedit" and click ok.

Windows Regedit

Command Line Editing: The registry can be manipulated from the command line with the reg.exe utility. It is included in Windows XP and Windows Vista and can be downloaded separately for previous versions. An alternative location are the Resource Kit CD's or the original Installation CD of Windows.