I'm just now realizing/remembering that the computer came with 10.5 disc, which I still have, and will try that instead. Yes, I bought Snow Leopard later.
You should still be able to just use the Snow Leopard disc.
See if you can boot from the original 10.5 disc. If so, try booting from the 10.6 disc again (before you install 10.5)
Wonder if 10.5 will run iTunes 11
or if it will let me upgrade using my Snow Leopard disc....