Most often, the worst you had to do, was delete the preference files. Sane settings would be recreated by the software. Try that with a registery file.

Beyond that, AV software was the biggest cause of instability.

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I managed a whole network of Macs back in the System 7, Windows 3.11/95 days. Yes the Macs of that time where easy for newbies to use, but when the OS became unstable you ended up having to bin it and start again.