G5 iMac won't boot up?

PLEASE HELP - I had a problem with my G5 iMac this evening and had to shut down manually by pressing the power button on rear. Now when I restart, I get the confirmation tone and it gets as far as the grey Apple screen, at which point I get a constant spinning wheel. If left, the fans come on like a jet engine but it never gets beyond this point. I've tried booting it in safe mode, but to no avail. Anyone any ideas?

Have you recently installed a new security update?

roflitskyle said:

Have you recently installed a new security update?

Click to expand...

Not as far as I'm aware.

Can you boot from the install disk? If you can, use Disk Utility.
1. Insert the cd that came with your computer and boot while holding the c key
2. After your computer is done booting select Disk Utility
3. Click first aid tab
IMPORTANT: Don't click continue on the installer window, if you do you will have to reboot holding c key
4. Select your hard drive on the left hand side of the window
5. Click repair