I just nuked the preferences, except for Finder and Users, since none really mattered and some were even still there from apps that aren't on the computer anymore.

I also put the extensions into a separate folder to try and find the bad one, and even there I'll be able to erase some of them (disinvectant doesn't serve any purpose, nor does the minitel connection assistant since the minitel doesn't even exist anymore, etc.)

I'm amazed how well this computer still works. Using it somehow feels extremely satisfying...