I've heard around the Internets that there is a patch for this (wonderful, BRILLIANT) game that removes the necessity for an A ranking to get that special ending I'm hearing about (and which I have kept myself from viewing on YouTube). Please tell me (if) such a patch exists, and where to find it.

Yeah, you can download the patch here: http://www.cheatbook.de/trainer/callofcthulhutrainer.htm -> Call of Cthulhu - Dark Corners of the Earth - Games Trainer
However, it works here, at my PC, but when I go to my dad, him having an older, non-updated PC, it can't load the trainer.

dext, google 'call of cthulhu svcklead patch'

I already gave him the link...