Some versions of Mac OS X will "pull up the ladder" once they are installed on your Hard Drive. They make settings that may preclude the re-install of older versions.
Do an SMC Reset and a PRAM Reset to eliminate this pre-conceived notion of which Mac OS X is the minimum.
If no joy, you need to run a test to check further:
Power off.
Remove all your Hard Drives.
Power on and boot from the Installer DVD.
The rule now in effect is: "Draw an Icon for each potentially-bootable Volume in the machine".
If no Icons are drawn AFTER the resets, either your DVD reader is not working or the DVD is not a good enough version.
If you suspect DVD Reader problems, you could try a DVD Cleaner Disc to wipe off the lens a bit.
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Do you have another Mac? If so, which one?