Driver and service configuration is stored in a subkey known as a control set. Because this configuration is important to system integrity, Windows maintains multiple copies of the control set. In most cases, there are three versions of the control set. If your system will not start because the current control set is corrupted, you can restore the last known good control set from a startup option.

The status of the various control sets is recorded in the Registry key HLM\SYSTEM\Select.

HINT:
You can study the difference between control sets by exporting each as a REG file and then comparing them with WinDiff.

On a workstation, if Windows does not start a troubleshooter will try the startup option to select the last known good control set. You may wish to have servers automatically go to the last known good control set if they do not start correctly. Windows can automatically restart using the Last Known Good control set if successful boot criteria are not met. To do this

1. Decide on a service or driver that must start at boot
2. Locate the subkey for the service or driver at HKLM\System\CurrentControlSet\Services\ServiceName
3. Set the ErrorControl value to control the system when the service does not start
   • Value 2 to reboot to last known good control set
   • Value 3 to halt the system

Although it is common to logon to workstation after a configuration change, this is not always the case with servers. Many servers are managed remotely and rarely have an interactive logon. There is a problem updating the Last Known Good control set on servers where there is no interactive logon after a configuration change. Windows supports a few custom boot verification methods that will update the Last Known Good control set without requiring an interactive logon. Programming or Registry configuration can ensure the copying of the current control set whenever the following programs start.

• Specialized programs
• Any selected service or driver that starts at boot time
• Bootvrfy.exe service from the Windows CD