If the computer will not boot to the DVD install media to allow you access to the Recovery Environment, I'm not sure what might be going on. Try to make an effort to normalize your hardware environment as much as possible, disconnect any external USB drives and other devices that you aren't using. Keep it as simple as possible, keyboard, mouse, monitor.
Have you been inside the case lately? If possible make sure your connections are all intact, SATA, IDE, addon cards, memory modules seated properly, etc. All fans, including power supply spinning properly?