My IMAC 20 core duo 2.4GHz 1GB 320GB DVDR HD2600 Pro 256 MB (dont know what all that is!) has been a little slow, particularly online, but more importantly has suddenly started making a very loud fan like noise when ever it is on! The thing is, this computer is virtually brand new, two months old. So, can anyone advise me on how to fix this problem?

Thank you, i have tried blowing air in and no positive effect. I find it hard to believe that it had collected any dust, but to eliminate the possibility i tried that.

The other stuff on that thread is useful but how do i test for any of that?

How do i know if its a hardware or software problem?

Did you install Leopard? Did this begin after you installed? I think the chances that it is something physical with the fan are probably low if it is only 2 months old. Are you certain it is a fan noise? Could it be the hard drive? Does the sound remain at a constant level at all times, or does it vary based on time of day, or what you are doing at the moment?

its pretty loud so becomes very annoying so i am not really using it.

I did install leopard a couple of days ago and it was ok but now, clearly is not.

I cant be certain of the noise, i dont know what a hard drive sounds like.

When i turn it on it immediately sounds like a hairdryer.

The little i have used it the sound sounds fairly constant.

Wow. On the one hand, my first guess was that it was the hard drive, especially since it started after installing Leopard. A hard drive is a lower sound, a little bit like a cats purr - not like a hairdryer!
I'm definitely no Mac expert, having made the switch myself only recently - but I believe its worth a shot to:
Leave your iMac on for a while -- you wrote that it sounds like this when you turn it on. Most modern operating systems, OS X included, do many things in the background, like maintenance. If you are interrupting that process by powering down, it will simply pick up where it left off or begin again when you power up. If you can, leave it for a few days powered on even if you are not using it. The tasks it performs will involve the hard drive a lot, causing not only that noise, but the fan as well to keep the computer cool.
I think if it hasn't reached a period of stable relative quietness after a few days, then you may have a hardware issue.

thanks, i'll give it a try.

It seems that this has occurred to others.

The solution i have worked straight away.

I had to shut the mac down, and unplug everything.

Then plug it back in and hold the on button on for approximately 10 seconds until it beeps.

Then have to hold down Option+Command+P+R while it reboots.

This apparently does some sort of power resetting.

Either way it solved my issue.

Thanks for all your help.

Thanks for posting your solution! Glad you were able to resolve it.