Startup with original install disc

I am trying to reinstall the operating system on my Mac Mini 10.4.11 from the original install disc. With the "C" key depressed and the install disc inserted, I start up the computer; after a few seconds the install disc is ejected, and the startup is completed with the previous OS X. The install disc icon does not appear in the finder. I would appreciate some advice.

Make sure disc is clean and not scratched.
Will the disc load from inside OS X?
Is your optical drive working?
Try holding the option key instead of the C key.

Thanks for your help, bobtomay. I tried your 3 suggestions, but none of them worked. Appreciate your thoughtfulness anyway.
sdbill2

Only had 2 suggestions. The other 2 are questions.

Will your disc load (can be seen) while you're booted up into the OS?
Is your optical drive working?

No, the install disc will not (cannot be seen).
Yes. Other CDs will load, so the optical drive is working.

Shut the machine down and unplug it. Take a can of air and give it about 3 quick shots in different directions inside the slot. Wait about a minute. Then boot up and give it a try again.