DVD/CD drive will not boot live CD's

OK... I built a computer about 2 years ago. Just a spare computer to play with so I didn't have to use my main one. I built it, put Ubuntu on it, and it was fine for a long time. I got a new main computer and could no longer us my KVM to switch between the PC's. My new computer does not have any PS/2 ports so I needed a USB KVM. Long story short, USB KVM's suck and I can't do that because not one I bought worked. Luckily my Samsung 226BW 22" LCD has dual inputs so I just stuck one on each machine and I have two keyboard and two mice. Before that however, I tried to dual boot my new Vista machine with Ubuntu 7.04. That did not go well, GRUB overwrote my Vista bootloader and I had to rewrite that then. So right now my Ubuntu HDD is messed up. All that is on there is the (I think) Vista bootloader. So I wanted to put the Ubuntu HDD back into my other machine (not the main one) and I would just have to suffer through having all the different peripherals. After I put the HDD in I made sure it would boot to CD and I put in my SimplyMepis boot disc. I tried my Feisty boot disc and my Kubuntu 6.06 disc and none of them work. This machine will not boot to CD and I have it set to! I don't know what to do right now it is an oversized paperweight. Please help!