Mac OS X 10.5 Leopard installation system requirements
Installing Mac OS X 10.5 Leopard
Mac OS X 10.5 Leopard Installation and Setup Guide
After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update
Does your DVD look like this?
A gray disc from another model Mac won't work.
Does it show as a boot choice if you hold alt key at bootup?
Might it be this?
Firmware password protection in Mac OS X ...
It would block usage of all the startup keys, like C, N, T, D, CMD+s, CMD+Option+p+r, CMD +v, Option boot will show a lock, and Shift, as well as booting from anything but the Hard Drive.