Added new hard drive; can't boot

I advised my parents to install an additional hard drive on their computer because they were running out of space. So when they bought a new 80GB drive, I of course installed it for them. I connected it properly and set the jumpers so that the original 20GB drive was the master, and the new 80GB drive was the slave. Then I turned on the computer. The monitor was black for a while, then got to the Gateway screen and stopped. I tried every possible way to connect the drives, I switched the jumpers around, I used different cables, I connected the cable in different ways, every time it either got to the Gateway screen, or didn't get anywhere and the monitor shut itself off. So I took the drive out and put everything back the way it was, and still could not get past the Gateway screen. I'm completely stumped here. The computer was running Windows XP Home Edition, has 512GB RAM installed, and a (I think) 2 GHz Pentium IV Processor.

Another thing I should probably note, when I went over to put in the new hard drive, I noticed the tower was REALLY hot. I put my hand on the back and nearly burnt myself. I'm going to clean it and install a new fan, but I don't think it will help with the hard drive situation, as it was hot like that the whole time, and when I initially came over it was working just fine.

Thanks in advance to anyone who can help!

fix the cooling issue first. once thats done and temps are within a normal range, copy any data needed from the master drive in another computer, then format or use the recovery disks.

Bios Settings?

Thanks everyone for your help. My uncle managed to fix the problem. He booted from a DOS boot disk and then formatted the new drive and installed Windows on it. Seems like such a simple solution for all that aggravation. Such is the computer world.