ok I recognized the jumper switch things on the hard drives. I took them out by using a paperclip in a creative way. After I did that, the original hard drive still doesn't work. However, now the computer detects the 2nd drive when I put it in as the main one. But to install Windows on it, I would have to reformat it. (dunno why that is)
The drive from the other computer (the third drive) also still didn't work for some reason, BUT, Windows 2000 setup detects it during setup. So I thought, what the hey, I might as well reinstall Windows on it. I dont know WHY it would need to and why it didn't work, since it already has Windows on it, but I went ahead and did it.
..and that went surprisingly well other than the fact that now the highest resolution I can choose is 800 x 600, and I am stuck with only 16 colors. I made this post on this machine.
Maybe next I'll put the 2nd drive in in the 2nd slot. If that works and I have all my old data then I would have basically fixed my computer except for the 16 colors problem.