Do you see the jumper pins on the CD/DVD drive next to the IDE cable connector? Are there any jumpers on it now?
Is there a label on the CD/DVD drive showing Jumper settings?
K, so you will NEED an image of 10.4 or 10.5 retail installer in a dmg to work with. I'm guessing you don't have access to another Mac? It might be tricky to get a dmg of 10.4 or 10.5 without a Mac, but I'll leave it to you.
Assuming that you get a working image on a USB, and you boot from it, what then?