IMAc, restarting issue

I would advocate using either Disk Warrior (better) which runs from its own disk, or using the original System Disk to boot (start up while holding down the C key), then locate and run the Disk Utility (free, but not as good). If you have Leopard, then you can also start in safe mode - try starting while you hold down the Shift Key. That will disable most 3rd party extensions and perhaps eliminate the error you have. Then run Disk Utility. Another option is to try starting while you hold down the Option Key, and then selecting the other partition shown, which is the repair partition.

Good Luck!