Try one of the other methods to choose the boot disk.
Here's a couple:
Leopard DVD in the drive, choose the 10.5 disk from your Startup disk pref pane, and restart from there.
Restart with the DVD in the drive, while holding your Option key, and choose the 10.5 DVD from the icons in the Option-boot screen. Click on the right-facing arrow to continue with the boot.
One more - restart, again with the DVD in the drive, and hold the C key until your see the rotating gear. Should continue to start to the disk doing that.
There's a lot of ways to do this, so come back if none of those work.