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"Last Known Good Configuration" is a recovery option Microsoft built into all versions of Windows, and can be a valuable asset when attempting to recover a PC that isn't working correctly. Last Known Good Configuration is found in <a href="https://neosmart.net/wiki/advanced-boot-options-menu/">the Advanced Boot Options menu</a>.
<h2>About "Last Known Good Configuration"</h2>
<h4>Background information about the Windows Registry</h4>
Windows stores almost all of system-related configuration and settings in the Windows registry. The Windows registry is split into 4 primary sections: <code>HKEY_CURRENT_USER</code> which stores settings related to the logged-in user account, <code>HKLM\SAM</code> ((<code>HKLM</code> is an abbreviation you'll see often in discussions pertaining to the Windows registry, and it stands for <code>HKEY_LOCAL_MACHINE</code> and contains global settings regarding your PC.)) which stores information about Windows usernames and passwords, <code>HKLM\SECURITY</code> which contains settings related to the security configuration of your machine, and <code>HKLM\SYSTEM</code> which stores all global settings for your PC.

The <code>HKLM\SYSTEM</code> registry hive, as mentioned above, contains most of the settings and configurations that Windows uses to determine what drivers to load, which settings to use, what policies are configured, etc. Inside <code>HKLM\SYSTEM</code>, all configurations and settings that have to do with your hardware, the drivers to communicate with that hardware, and system services are found in <code>HKLM\<code>CurrentControlSet</code></code>.
<h4>CurrentControlSet</h4>
To protect against system errors and provide an easy recovery option, each time Windows is successfully booted, the old <code>CurrentControlSet</code> and all its contents is copied and set aside. In reality, <code>CurrentControlSet</code> is actually a shortcut/pointer to one of <code>CurrentControlSet001</code>, <code>CurrentControlSet002</code>, or <code>Clone</code>. When your computer first starts up, Windows copies <code>HKLM\SYSTEM</code>\<code>CurrentControlSet</code> to <code>HKLM\SYSTEM</code>\<code>Clone</code>. <code>CurrentControlSet</code> is then "pointed" at <code>CurrentControlSet001</code>, to which it serves as a shortcut. Any changes made to <code>HKLM\SYSTEM</code>\<code>CurrentControlSet</code> are in actuality being made to <code>CurrentControlSet001</code>.
<h4>Last Known Good Configuration</h4>
This finally brings us to the topic of the day: Last Known Good Configuration. If changes made to <code>CurrentControlSet</code> end up causing Windows not to work properly, the configuration that was used last time for Windows is still available in the backed-up copy. When Windows fails to start, it is possible to to request Windows boot into "Last Known Good Configuration." Here, instead of pointing <code>CurrentControlSet</code> to <code>CurrentControlSet001</code> (as is the default behavior), Windows will instead make <code>SYSTEM\CurrentControlSet</code> point to <code>CurrentControlSet002</code> — which (since we used it before) we know is good and working.
<h2>Booting your PC into Last Known Good Configuration</h2>
Directions on how to configure Windows to boot into Last Known Good Configuration have been moved to a separate article, as this detailed description of how Last Known Good Configuration operates underneath the hood contains advanced details, knowledge of which is not required to actually make use of LKGC.

See <a href="https://neosmart.net/wiki/booting-into-last-known-good-configuration/">Booting your PC into Last Known Good Configuration</a> for directions on restoring your PC to a working copy of <code>CurrentControlSet</code> via <code>CurrentControlSet002</code>.