One particularly frustrating aspect of working with Windows involves making an incorrect configuration change that...
completely disables the OS. Fortunately, depending on the change that was made, it is sometimes possible to return Windows to a functional state with minimal effort. In this article, I will show you some techniques for recovering from various types of configuration problems.
The first technique that I recommend trying in an effort to roll back a catastrophic configuration change is to use the Last Known Good Configuration feature. The basic idea behind this concept is that when Windows boots successfully, it makes note of the current configuration, marking it as good. If a catastrophic configuration change were to occur and Windows is unable to boot, then the system still has a record of a configuration that is known to be good. A boot menu option will allow you to boot Windows using this configuration and restore the known good configuration.
To use the Last Known Good Configuration, simply power up the computer and begin pressing the F8 key repeatedly until the Windows boot menu appears. Then select the Last Known Good Configuration option. If you are able to boot Windows using the Last Known Good Configuration, then there is nothing else that you need to do (i.e. Windows should boot successfully on the next attempt without you having to do anything special).
In case you are wondering, the Last Known Good Configuration option reverses the most recent system and driver changes within the hardware profile. The system and driver configurations are returned to the state they were in at the time of the most recent successful boot. When you boot using the Last Known Good Configuration, Windows also restores the registry settings found beneath the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet key.
Microsoft recommends using the Last Known Good Configuration option as the first technique for repairing a system that won't boot properly. However, some seasoned pros prefer to try and boot the system into Safe Mode and attempt to manually correct the problem.
In case you aren't familiar with Safe Mode, it is another boot option found in the Windows boot menu. Often, you can boot an unbootable system into Safe Mode because Safe Mode loads Windows using a minimal set of drivers. That way, you can make any necessary configuration repairs and then boot the system normally.
The disadvantage to using Safe Mode is that it is only effective if you know how to fix the problem that rendered the system unbootable in the first place. The Last Known Good Configuration feature, on the other hand, usually fixes the problem even if you don't know what caused it.
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Another thing to be aware of regarding Safe Mode is that Windows does not consider booting into Safe Mode to be a successful boot. This means that if you attempt to fix a problem manually by booting a machine into Safe Mode, but you aren't able to correct the problem, then you still have the option of using the Last Known Good Configuration feature. Since Windows doesn't consider a Safe Mode boot to be a successful boot, it does not overwrite the Last Known Good Configuration when you boot into Safe Mode.
The Last Known Good Configuration option is one of the safest and easiest techniques for repairing an unbootable system, but it will not work in every situation. For example, if you accidentally disable Windows by making an incorrect modification to the registry, then this technique probably won't help you (although there are sections of the registry that will be repaired). If you run into such a situation, then usually your best option is to restore a backup or reinstall Windows.
As I said before, the Last Known Good Configuration feature is only one way to recover from catastrophic configuration changes in Windows. In an upcoming article, I will show you how to manually roll back device drivers and use System Restore to roll back configuration changes.
Brien M. Posey, MCSE, is a Microsoft Most Valuable Professional for his work with Windows 2000 Server, Exchange Server and IIS. He has served as CIO for a nationwide chain of hospitals and was once in charge of IT security for Fort Knox. He writes regularly for SearchWinComputing.com and other TechTarget sites.