So today, I tried putting the old hard drive into computer "B", my dad's computer. After I opened up computer B and unplugged the wires from the master drive so I could have room to put in the slave drive (computer A's hard drive). When I connected everything again and put everything back together, computer B's fans would instantly turn on, the power light would blink, the CD/DVD optical drive would blink and the speakers would make a thumping noise.
I took out computer A's hard drive and tried turning on computer B with just B's hard drive, but the same thing happened.
So, now I don't know what else to do. Does anyone know what problem I have and how I could fix it?