Unless it is a home built computer, computers have stickers from the OEM. That might provide a clue to what operating system the computer was running..
No sticker. I believe my brother built this puppy. Another (if sentimental) reason to put a bit of effort into it...
But again, even if the HD was dead or pulled, I should be able to get an initial VGA or HDMI screen, unless the BIOS (or at least the part of it that controls the POST) is shot...?
My daughter’s friend who likes to mess with old computers is out of the area for another week, but, if something fairly “simple” doesn’t turn up, to him this will go... My buddy with a DVI input monitor should be back this Sunday.