External CD Drive OS Installation

I am currently using a Macbook with a defunct CD drive on OS 10.5(Leopard). The external CD drive I have connected (via 2 USB ports) works fine when the computer is started through OS 10.5 and reads/writes content fine. However, trying to boot from the disk is unsuccessful because it will recognize the name of the disk as "WIndows" and it asks to "Insert a disk and hit any key to continue".

How can I get the computer to boot the disk from the external CD Drive, without asking me to insert a disk into the internal Disc drive?