When trying to boot from the OS X install disc (Snow Leopard) my Macbook will just eject the disc and boot normally from the installed system on the hard drive.
Once booted, the disc can be read fine by the computer, but when trying to restart in order to make a fresh install of the operating system, the computer will just again eject the CD and boot from the hard drive.
Holding the C key when during startup to force booting from the CD drive wont work, and the CD is ejected again. Even holding the ⌥ (alt or option) symbol during startup will cause the CD to be ejected, and the only boot option shown is the hard drive.
(Note, as I've found threads with similar questions elsewhere on the internet, all with long discussions but none with an answer, I'm documenting here the solution that worked for me.)