It would not surprise me if there is some forensics software or system that does this...

20 year old downloaded videos - that's like 480p xvid avi? I vote for re-downloading. I have had some stuff like that flying around from back in the day. Whenever I have wanted to watch one, I lasted a few seconds until I decided to dump it and get a better version. I would keep rarities, of course, but otherwise it might even be quicker to re-download than to mess with disks.