HKEY_CLASSES_ROOT contains information about file associations and data associated with COM objects. As shown in Figure 23.27, this is the same data as in the Classes subkey under HKEY_LOCAL_MACHINE\Software.
Figure 23.27 HKEY_CLASSES_ROOT and the HKEY_LOCAL_MACHINE\Software\Classes subkey
The sole purpose for HKEY_CLASSES_ROOT is to provide compatibility with the Windows 3.1 registration database.
HKEY_CLASSES_ROOT contains the data that associates file types (by filename extension) with the applications that support them. Do not change file associations by using Registry Editor. Instead, to change a file association, in Windows NT Explorer, from the Options menu, click View, then click the File Types tab.
For more information about HKEY_CLASSES_ROOT, see the OLE Programmer's Reference in the Windows NT 4.0 Software Developer's Kit.