Q. Where do entries in the HKEY_CLASSES_ROOT registry key come from?

The HKEY_CLASSES_ROOT registry key contains data that associates file types with programs and contains configuration data for automation, like Visual Basic programs and COM objects.

In Windows NT 4.0 and earlier, the HKEY_CLASSES_ROOT registry key was just an alias for HKEY_LOCAL_MACHINESOFTWAREClasses.

Newer versions of Windows NT support per-user class registration, merging the content of HKEY_CLASSES_ROOT from both HKEY_CURRENT_USERSOFTWAREClasses and HKEY_LOCAL_MACHINESOFTWAREClasses.

NOTE: If a Value Name appears in both HKEY_CURRENT_USERSOFTWAREClasses and HKEY_LOCAL_MACHINESOFTWAREClasses, the value from HKEY_CURRENT_USERSOFTWAREClasses is posted to HKEY_CLASSES_ROOT.

NOTE: If you create a sub-key in HKEY_CLASSES_ROOT, it will also appear in HKEY_LOCAL_MACHINESOFTWAREClasses.

NOTE: If you create an entry in HKEY_CLASSES_ROOT and the sub-key already exists in HKEY_CURRENT_USERSOFTWAREClasses, then the entry will appear in HKEY_CURRENT_USERSOFTWAREClasses, else it will appear in HKEY_LOCAL_MACHINESOFTWAREClasses.