HKEY_CLASSES_ROOT : – data on DLL files and applications is stored.
HKEY_CURRENT_USER – current user’s individual customizations are stored and tracked.
HKEY_LOCAL_MACHINE: – all of the software installed on the computer. Virtually anything that the operating system might need to know about a particular application is stored here.
HKEY_USERS: – all user data is stored. If multiple people use the same computer, all their user profiles are stored here. When a user logs on to the computer, the data is written to HKEY_CURRENT_USER.
HKEY_CURRENT_CONFIG – This key points to the current computer hardware configuration in the collection of configurations stored in HKEY_LOCAL_MACHINE. This enables the use of multiple computer hardware profiles.