Mac keeps re-starting but does not boot up

Hello My fellow Mac chums,

I have a problem with a g5 imac running Tiger, I turned it on this morning
and it just keeps booting, ie spinning wheel and the chime, but not actually starting up. I've tried shift key on re-start and I get the hard drive logo with the arrow pointing up, when I click on it, I then get the spinning wheel and it goes through the cycle of booting up turning off and booting etc.

I've tried apple shift r and p and wait the the 3 chimes release the keys and get the same problem. It's a real pain.

Does anyone have clue what to do.

Kind Regards,
frustrated reprochappie