Hkey_Classes_Root             

This key displays information about OLE and associated mappings to support drag-and-drop operations.

Hkey_Current_User           

This points to a branch of Hkey_Users for the currently logged on user.

Hkey_Local_Machine         

This contains computer specific information including installed hardware and software. This is the one I tend to spend the most time in.

Hkey_Users                          

This contains information about users that have logged on to the computer.

Hkey_Current_Config       

This key points to a branch of Hkey_Local_Machine Config and has information such as display fonts and printers.

Hkey_Dyn_Data                  

This key points to a branch of Hkey_Local_Machine that has information about Plug and Play devices.