It is a little hard to believe the system will not boot into a bootable DVD or Flash Drive. Do you feel confident you are hitting the necessary keys during boot to allow booting into the bootable media? Are you getting the "Boot to DVD, hit a key" type message? You could always try a Linux live CD to boot and check your system.
But what happens when you try to boot? Does the screen go dark, are there any beeps or other indications of what might be happening?
Any system info for us, and anything been done to your system like adding software or hardware?