Big Bird may be right about the overclocking. On some Macs it changes the gestalt ID of the system when you do certain things that deviate too far from factory parameters. The installer on your OS CD is probably looking at the gestalt ID of your system and isn't finding it on it's list of approved models and is rejecting it. You might have to change the gestalt and use a program like Wish I Were... to trick the installer into thinking you have a known Mac for it to install to.