Last Known Good or LKG is a term used to define the configuration of Windows NT or Windows 2000 at the last successful logon. One would select Last Known Good Configuration during bootup if new software of some sort had been installed that prevented the OS from loading.
I have never seen LKG work.

I do have a co-worker that once took a machine out of a domain and placed it in a workgroup, booted up & realized he didn't know the local admin password or what any of the local accounts were, did an LKG and he claims the machine was back in the domain & he could log in. I don't know for certain if this is true.