Constantly ejecting OS X install disc when trying to boot

Solution 1:

Restart the computer, then press and hold the ⌥ key just after you hear the startup chime. Even if the install disc is ejected, and you are only shown the hard drive as your unique boot option, at this moment you can again insert the disc, and wait for the computer to read it. If everything goes fine, you will now see the option to boot from the CD.

Solution 2:

I shut down the computer, unplugged anything that was attached to it (peripherals, power cord, etc.), held down the power button for more than 10 seconds, plugged power cord, keyboard and mouse, turned the computer back on, waited for main screen, and inserted DVD. It took a while for it to read and install but finally got it to work.

Got it from this post by Rhys Pritchard on the Apple support forums.