After installing SATA optical drive my computer cant POST

¡Hi everybody! I needed to ophcrack a computer because it's owner forgot the admin password. The computer didn't have any optical drive, so I tried to use an external, usb one. The pc wasn't able to boot from the external drive so I temporally installed a SATA one. At first, the pc turned itself off when POSTing, but after a few tries I got the computer to work perfectly, I was even able to boot from the ophcrack cd, but since it was version 2 and the computer runs Windows Vista I wasn't able to get any passwords. Later on I downloaded the latest version (3.8), burned the CD and restarted in order to boot from the ophcrack cd, but again, the computer shutted down when the bios screen was showing. I tried I guess 5 times in a row and got nothing. I decided to unplug the drive and see what happened, but now when the pc turned on it wasn't able to show the BIOS screen. Everything seems to work normal: the leds behind the RJ-48 are always turned on, and the keyboard leds turn on when the computer first starts, later on they turn down and stay turned down even if I press the caps lock key... Any ideas?