24" Aluminium iMac won't start

Never seen this before - when it comes to the grey screen it start a cycle of icons - apple, no entry, a folder with a ? on it, all flashing constantly.
Have tried the pram and nvram, no change, also tried simple safe mode, also no change.
I'm going to dig out the leopard discs to try booting from there, but wondered if this is ringing a bell for anyone?

try using your OS disk to boot it up then use Disk Utility from the disk to verify disk and then repair permissions. after you boot the OS disk and hit english go to the menu bar and open disk utility.