cd drive ejects when bootup

any chance you are holding down your mouse button or does that not work now days?

hm....nothing in the drive? and isnt selected as a start up item? (you dont have windows installed on your machine by chance?)

try a SMC reset (unplug all cables to the computer; mouse keyboard monitor firewire/usb etc) for a few minutes then plug back in and then do a PRAM reset

COMMAND OPTION P R all at the same time right before the apple/grey screen appears. then it will reboot and chime again.

My G5 does that when I live my DVD-RW disc in there. It spits it out on startup. Is your CD tray empty or do you have a disc in it?