Try resetting the open firmware. Boot while holding down the Command-Option-O-F keys. At the prompt, type:
reset-nvram [return]
set-defaults [return]
reset-all [return]
At this point the computer will reboot. Have a 10.3 CD in at the time and see if you can boot from it by holding down the C button. If not, you may need a new combo drive as has been suggested.
(you also could try resetting your motherboard, but I don't think I would do this without first reading up on the subject and seeing what the effects/reasons for this are)