How can I reinstall system 8.6 to get rid of some major trouble (including desperately slow computer) with a Performa 6200 (603, 75MHz) without losing all my extensions and preferences. I have been using this computer for work for the past five years and just don't remember what I installed into it as far as extensions and patches etc. Also, would it be feasible to install an older version of the system software (like 7.5) to make the computer work smoother?Thanks for the help.

You could do a clean install to a new system folder, and then migrate your old control panels, extensions, etc. to that folder as needed. Once you know that you've got all the necessary components, you can trash the "fluff" that's left. Of course, the 8.6 install itself will dump a lot of useless fluff onto your system (Game Sprockets, 8 million printer drivers, etc.), which should also be weeded out to save you some space and resources. As far as OS versions go, 8.6 is pretty stable; personally, I'd stick with it.

Doesn't the install have a custom install too? You could use that and install only what you know will work and you know you need. Anything you forgot about will still stay there anyway.