This article was previously published under Q101790
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A collection of configuration data needed to control system boot iscalled a control set. A single control set is equivalent to, in theMS-DOS/Windows environment, the CONFIG.SYS file, some of theAUTOEXEC.BAT file and part of the WIN.INI file. The last known goodcontrol set works by keeping a backup control set of the lastsuccessful boot. The rules for determining if the last boot wassuccessful, and therefore good, are:
No system critical errors involving the failure of a driver or system file.
A user can log onto the system at least once.
What is the Last Known Good Control Set Good For?
The following three examples demonstrate what the last known goodcontrol set is good for:
You install a new device driver and restart Windows NT. The system stops responding (hangs) when you start (boot) the computer. The last known good control set will enable you to boot because it does not contain any reference to the new, faulty driver.
You install a new video driver and are able to restart the system. However, you cannot see anything because the new video resolution is incompatible with your video adapter. In this case, do not try to log on by entering the correct keys. If you turn off and restart your computer, the last known good control set can be used.
You accidentally disable a system critical device driver (such as the SCSIPORT driver). Windows NT is not be able to boot and reverts to the last known good control set.
What is the Last Known Good Control Set NOT Good For?
Any problem that is not related to changes in control set information will not be corrected by reverting to the last known good configuration. This includes information like user profiles and file permissions.
Any change introduced more than one successful boot ago cannot be backed out, because the change will have been copied to the last known good control set on the first successful boot after the change was made.
The last known good control set is a backup and restore facility for the Registry; it does not support switching between configurations (docked and undocked laptops, for example).
The following are three examples where the last known good control setis not helpful:
Boot failures caused by hardware failures or corrupted files.
If you copy a new driver over the top of an old one, and the old one is already active, then the configuration will not change; switching to the last known good control set will not undo anything.
If Windows NT boots, a user logs on, and then Windows NT hangs, the last known good control set will not help because it has already been updated to the current control set.