Recently, during a virus-cleanup, it became apparent that the PC would not boot unless a specific Windows Registry key was edited.

Up until this point, it was (incorrectly) just assumed that you had to be in the Windows operating system in order to access the registry. This is not the case! The particular virus afflicting the client's PC caused the system to log off immediately after they logged in. This occurred in regular mode, safe mode, as well as with all user accounts.

The system registry is located in %systemroot%\system32\config and is divided into several separate branches

You can load the registry on another computer, edit it, unload it, and (in this case) solve problems using this method