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If your willing to lose data on the HD... you can reinstall the OS on it via Firewire if you have another Mac. Turn on the Mac that doesn't want to load up... hold down the letter "T" and it will be in Firewire mode. Connect a firewire cable to another Mac.. the HD will load onto the 2nd Mac.... then insert your OS X installation disc, and install it onto that HD that's connected via Firewire. You can do an "Archive and Install" to keep your settings. Or do a fresh install.
 
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