I would also suggest repairing the disk, but i'm not sure how much this will help if you have tried booting from another system with the same results. It would be helpful, however, as you could find if you can still boot off a CD. Are you sure you are booting from the secondary system, and the secondary system defiantly works?
Have you tried reseting the SMU? (no PMU)
http://docs.info.apple.com/article.html?artnum=303319
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