Trying to upgrade to Leopard from 10.4.11

Hi

I have an I Mac G5 running 10.4.11. I have a full retail install disc for leopard 10.5.6. When I inserted the CD it opened up, I clicked on install, entered my password, then clicked restart. The computer restarted, the apple icon came up then it disappeared, and a blue screen appeared. The disc was spinning in the drive and the rainbow ball was spinning on the screen. It was doing this for a long time, so I had to his the power off button, then restart and eject the disc. I tried another leopard disc and the same thing happened. Any ideas what could be wrong?

Thanks