The Windows XP registry is composed of 5 main branches. These separate the data stored in the registry into five distinct areas. Within these branches are keys, equivalent to folders, which hold subkeys and values.

HKEY_CLASSES_ROOT: This branch holds all the information Windows needs for internal functions such as OLE (Object Linking and Embedding, the ability to copy and paste between different programs and documents) and drag-and-drop. It is also the database for file associations, which is how the OS determines what program opens when you double click a specific file type. This branch is by far the least user-friendly and the least edited of the six. Best to leave it alone then.

HKEY_CURRENT_USER: Contains a variety of configuration settings for the user that is currently logged in only. If you have made customizations to Windows, such as a new desktop, or a font change, it will be stored here. Any Windows or application setting that is specific to your user can be found within this branch of the registry. As such, there are many customization options here. HKEY_CURRENT_USER is actually a sub-set of the HKEY_USERS branch, specific to the currently logged in user only.

HKEY_LOCAL_MACHINE: Probably the most edited registry branch. This contains configuration settings that are specific to the computer and the software installed on it, but independent of users. As such, any changes made here will apply to any user that is logged in. The SOFTWARE key within this branch contains configuration settings for all installed software, including Windows itself. Look into SOFTWARE \ Microsoft \ Windows \ CurrentVersion for many configuration options.

HKEY_USERS: Like the CURRENT_USER branch, this contains individual configuration settings specific to individual users. However, it contains these options for all users that have been created. The users are identified by their SID (Security Identifier) number. This unique value is assigned to each user at creation.

HKEY_CURRENT_CONFIG: This is connected to HKEY_LOCAL_MACHINE, in much the same way that the CURRENT_USER branch connects to HKEY_USERS. It specifies hardware and software settings that apply to all users, for a given hardware profile.

Inside each of these branches are 'keys' represented by folder icons, that hold subkeys and the values that make up the registry's data.

The values themselves tend to be one of five main types: