Refers to a set of registry keys in Microsoft Windows NT and Windows 2000 that contains configuration information used for system startup. Control sets define certain aspects of the Windows NT boot process to allow Windows NT to boot up successfully. Up to four control set subkeys are located under the HKEY_LOCAL_MACHINE\SYSTEM registry key, including the following:
The Select registry key under HKEY_LOCAL_MACHINES\SYSTEM identifies which of the control set keys corresponds to the current, default, failed, and Last Known Good configurations. If the current control set can’t start the system, you can press the Spacebar when indicated during the boot process to select the last known good configuration, which is the last control set that worked for sure.