I did a search for overheating Macs and found some software called smcFanControl I downloaded it and installed it and set the minimum fan speed for the hard drive to 2750 and the cpu to 2000, I left the optical drive fan alone for now although I may change it when burning discs because they do seem to come out hot when burnt Hmmmm :-D.
I have been running my iMac at these settings for the past two days and everything seems ok, The top left of my iMac is now cool although the back is still a bit warm but not causing any problems so far.