Try resetting the SMU.
Your iMac must be shutdown prior to trying this.
Unplug all cables from the computer, including the power cord. Wait 30 seconds.
While pressing and holding the power on button on the back of the iMac, plug the power cord into the back of the iMac. Release the power on button. Wait 10 seconds, press the power on button again, and if the SMU reset was successful, the iMac should restart.
Did not work.
When the fan was originally blasting away, lots of dust came out of it. I managed to keep trying to boot it, and it booted sucessfully. Copied some files to a hard drive. Used disc utility which said need to repair hard drive, but 'repair" button was not available. My OS install disc will not boot properly to allow me to use its disc utility to perform a repair. Not sure this is theproblem though. Now when I try to start, the fan runs, nothing else happens. Yuk. I want my old Color Classic.
Disk Utility wouldn't 'repair' the disk because you can't use your startup volume to repair itself.
You need to restart the computer from an external HD loaded with the same OS you're using, and Disk Utility, or from the original install disks, or from a retail version of OS 10.5.
It could be that the install disk you have is not the proper install disk for your model of iMac.
If you have a 'black' OS 10.5 retail disk, put the disk in the slot while the computer is off, and restart the computer while holding down the 'C' key. Continue holding the 'C' key until the OS finishes loading.
Results: The Apple tech store says this model is notorious for this problem, and it involves the capacitors on the logic board. We opened it up, and the capacitors were visibly expanding and twisting on the board. At one point, he stated, they will expand and explode... sounds dramatic, but explains the flakey performance and lack of booting. I'd say this problem makes the fan take over on high drive mode to cool . . . and locks out the functioning of the processor in the process. Hard drive is not a problem.
There are no parts available other than used parts market, and the procedure is maybe works maybe not. I can't imagine replacing all the capacitors without something else happening. They are probably attached with two wires as he stated, but tricky to replace. I will have to buy a new imac, they come out in 2 weeks or so. Good timing at least.
As for software, maybe leasing at 30 per month for Creative Suite would work. A new imac won't use the cloned drive from the PPC architecture. Of course, why is that no surprise....
Ce la vie...