g5 10.4.11 ppc bootup problem

[reallyoldnewbie]

apologies if this is somewhere among all the threads- haven't looked 'everywhere' yet. I cant get my old imac to bootup- it will chime. I have done the pram clearing, stuck in my old original install disc. tried the c key thing, it will only go into the grey screen with the spinning wheel and the fan runs on high speed. i tried the safe mode shift key thing, i get a kernel error messages with the "must restart" screen. i have tried many key combos as advised in various places- this all started after i downloaded an old calibre app for this setup from sourceforge- guess it was a bad idea. and now my install disc is stuck in the drive... the kernel errors are for extensions, i understand? anyway- if anyone out there can help me boot this up somehow- sure would be appreciated... I have loved that old desktop for a long time- still working great - till yesterday... the notebook i am using is a nice old 13" intel core duo running snow leopard- still a great unit! thanks for being here!