Looking in;
HKEY_LOCAL_MACHINE\SYSTEM\Select
you'll find the D-Word values for
"Current"=dword:00000001
"Default"=dword:00000001
"Failed"=dword:00000000
"LastKnownGood"=dword:00000002
CurrentControlSet is volatile and will always be an image (at boot) of what's defined in ControlSetx where x = the value of "Current"
Choosing last known good boots the system with the control set that last successfully booted your system. Control sets contain system configuration information such as device drivers and services.
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