The Windows Registry contains a root key titled HKEY_LOCAL_MACHINE, or HKLM. The HKLM root key contains settings that relate to the local computer. In Microsoft Windows XP and prior, there are four main subkeys under HKLM: SAM, SECURITY, SOFTWARE, SYSTEM. There is also a fifth subkey, titled HARDWARE, which is created on-the-fly and is not stored in a registry file. In Windows Vista and 7, a sixth subkey is present, titled BCD.
The SOFTWARE subkey is the most commonly accessed registry key, as it contains the settings for Windows and the software programs installed on the computer. When a software program is installed, data is written to the SOFTWARE subkey to store application-specific settings. These settings are loaded into the computer memory each time the operating system starts, so that when a program is loaded by the user they do not need to configure the program again.
Also see: Windows Registry