I've had the same problem several times on my G5 iMac with iSight... on start-up it gets a black screen with a loud fan sound and won't do anything else. The problem seems to occur when it doesn't eject a CD before shut-down (power outage, pulled plug, etc.)
Here's the solution that has worked for me. Unplug all devices (USB drive, printer, etc.) except the keyboard... then reboot with the mouse button held down. The CD will eject and you get a clean boot.
Not sure why this occurs, but it seems that the computer gets stuck trying to boot from the CD and this clears the problem.
Hope that helps!