Why can't I boot from any source?

Hi! I have an iBook G3 500 MHZ and I have recently replaced the DC-in board and put in a newer hard drive from one of my dead iBooks and a slot-loading combo drive. My computer now won't boot from anything. I have tried my internal hard drive, which has OSX 10.3 on it, firewire target disk mode from 2 iBook G4's, and Mac OSX 10.4 Tiger installation DVD boot. When the iBook starts up the screen flashes, becomes black, and lights up again with a blinking ? and Finder icon. I start up again while holding the option key and the blue screen with my hard drive, install DVD, etc. comes up. I wait for the watch to go away and click the install DVD, and then the right arrow. The screen then flashes blue and starts finding the disks again. It doesn't boot. What is wrong?

UPDATE: I just installed Tiger on the iBook using Firewire Target Disk Mode and confirmed that the drive is just fine.