The ControlSet00n subkeys represent a control set for the system.
The numbered ControlSet00n subkeys, such as ControlSet001 and ControlSet002, contain control sets that can be used to start and run Windows 2000. Most systems have two numbered control sets, an original and a backup copy of a control set that has been used to start the system successfully, but the system can maintain as many as four control sets. Windows 2000 maintains backup copies so that you can undo configuration changes that might otherwise prevent you from starting the system.
Windows 2000 identifies the control set that was last used successfully, the current control set , and the default control set. These designations are stored in the Select subkey.
Windows 2000 automatically maintains the control sets and selects one of them when the system starts. It bases this selection on settings in the Select subkey and on the success or failure of system startup. When the system starts, users can also select the LastKnownGood control set during the boot loader procedure.
All the numbered control set subkeys, including the one that appears in the CurrentControlSet subkey, can have the same subkeys and entries. In this reference, these subkeys are entries are listed under the CurrentControlSet subkey.
The ControlSet00n subkey is a variable representing the numbered control sets in the HKLM\SYSTEM key. It does not actually appear in the registry.