in message
There are only 2 real hives: local machine and users. All the others
are *copies* (partial) of those root hives; that is, there are 2 root
hives and the others are pseudo-hives. HKCU shows the entries under
the HKU hive but only for the currently logged on user. HKR shows a
mix of entries from HKLM and HKU for the currently logged on user.
The HKCFG hive is a mix of System (used for HKLM) and ntuser.dat info.
There are only 2 real hives. The others are compilations made up from
those 2 real hives.
The SAM, Security, Software, and System registry files are all part of
the HKLM hive. The HKU user hive is taken from the ntuser.dat file
user the currently logged on user's profile path. The HKCFG hive is
compiled from the System and ntuser .dat files.
http://www.bleepingcomputer.com/tutorials/tutorial74.html
http://support.microsoft.com/kb/256986
http://en.wikipedia.org/wiki/Windows_Registry