How best to boot a sick G4 P'mac ?

Was at my father-in-law's today and his G4/400 (10.4.11) wont boot.  It 
hangs on the grey apple screen with the spinning wheelie thing.  I can 
hear the hard drive is whirring and clicking but it never boots up.  I 
had my Tiger and Panther install discs with me but restarting with 
either of those in the drive whilst holding the "c" key didn't work 
either - same hang on the grey screen.

Any thoughts on what the problem might be and what I should try next.  I 
was hoping a "repair permissions" might sort it but I am suspecting it 
may be more "terminal.

Thanks
--
JohnB