Have you tried the standard trouble-shooting stuff of seeing whether optical discs load and burn in a second admin user? If that doesn't work, try resetting nvram:
During startup, hold down command-option-p-r through at least 4 chimes, then release. Or use the open firmware method.
http://docs.info.apple.com/article.html?artnum=2238
USING OPEN FIRMWARE:
1) reboot while holding option-command-o-f (boot into open firmware)
2) type: reset-nvram (return)
3) type: reset-all (return)
4) the Mac should restart on its own, if the commands have been typed correctly.