The HKEY_CLASSES_ROOT Key

The HKEY_CLASSES_ROOT Key
The HKEY_CLASSES_ROOT root key contains information on all existing filename
associations and data associated with COM objects. However, in Windows 2000 and later
versions, including Windows XP and products of the Windows Server 2003 family, the
contents of this root key became more complex. Similar to Windows NT 4.0, the data
under this key references the Classes subkey of the
HKEY_LOCAL_MACHINE\Software hive, but, besides this, it also joins the contents of
the HKEY_CURRENT_USER\Software\Classes key. The latter key is actually a link to
the HKEY_USERS\<SID>_Classes keys, where SID stands for security identifier of the
current user.

Note As was already mentioned, the HKEY_LOCAL_MACHINE registry key contains
global settings applicable to all users who log on to the local system. The
HKEY_CURRENT_USER root key, on the other hand, contains settings that are
specific to the user who is currently logged on to the system. With a small number
of exceptions, user settings have the priority above general computer settings.
Therefore, if the file type or COM class is specified both under
HKEY_LOCAL_MACHINE and HKEY_CURRENT_USER, the OS will use the
setting existing under HKEY_CURRENT_USER.
The HKEY_CLASSES_ROOT key contains the data that associates file types (by
filename extensions) with specific applications supporting formats of these files.