No hard drive

What generation is the iMac?

HDDs are cheap to replace.

Do you have the Hardware Test Utility CD?

It should have come with your iMac

I may have one lying around, but it is for my old PowerBook, so it won't help you much

If you can't find your hard disk in Disk Utility, then it's a hardware problem (damaged hard drive). Take the computer to an Apple Store or reseller to get a new hard disk.

If you want to do it yourself, you can find the steps in iFixit. In this page, you can follow how-to's to replace your hard drive and you can get the hard drive, but you can also buy it in other pages, as OWC

If you have the Mac OS X Tiger DVD, it's a 2007 or older iMac. To run Apple Hardware Test, insert the Mac OS X Install Disc 1 and hold the D key while your Mac is starting. Remember to run an extended test, and if you see an error at the end, copy it here. See > http://support.apple.com/kb/ht1509

It's Mac OS X Tiger, so it's a Mid 2007 or older iMac. If the iMac is built with aluminum instead of plastic, it's the Mid 2007 one. If not, it's a Late 206 or older one

Hold the D key after the startup sound, and see if Apple Hardware Test starts loading

That error is referred to the hard drive fan. Take the Mac to an Apple Store or reseller to get it repaired

I'm pretty sure that's a hard drive fan problem. Have you tried to re-run Apple Hardware Test after taking it to the Apple Store? Maybe it was just an Apple Hardware Test error