Dell logo then blank screen with white cursor after boot.

My brother's got a computer that he's fixing for a friend. I'm not sure the exact model but I know it's a Dell and has a Core 2 Quad in it. After you hit the power button it shows the Dell splash then goes to a black screen with a white blinking cursor and stays there until rebooted.

We've set the hard disk as the first boot priority and even booted from CD drive with the recovery disk in. Neither cause anything different. When the hard drive was taken out and put in another computer, it works fine. When the HDD from the other computer is put in the Dell it does the same thing with the flashing cursor. We've made sure all the cables are secure, RAM is set properly, and reapplied thermal paste to the processor.

Does this sound like a motherboard issue, or is there something that we haven't tried yet?