So when you boot from the OS X Install disc, does it fully boot up from the disc into the installer and then freeze, or does it freeze in the process of booting? Either way, not a particularly good sign. Disconnect all external peripherals. Try another install disc perhaps. If simple troubleshooting like that doesn't help, your next step would probably be to remove any third party RAM. If that still doesn't help, I'd say it's likely a logic board issue. But you shouldn't jump to that conclusion before taking other troubleshooting measures.