From what I understand, you want to reinstall Windows Vista, boot your computer won't boot into the DvD.
Power your computer on. When the BIOS screen appears (the first screen to appear during the boot process), press the "Delete" key. This is the most common way to enter the BIOS settings on a computer, though some systems differ. If the Delete key does not access the BIOS setting on your computer, look for different instructions on the screen itself. For example, it may say "Press F10 to enter setup."
Using the arrows on your keyboard, scroll down to "Boot Device Select" and press "Enter."
Change the boot order, moving DVD drive to the first boot device.
Insert the DVD you wish to boot from.
Save your changes and exit BIOS. Look for on-screen instructions on how to save and exit, as this option differs between systems. Exiting BIOS will force your computer to restart. When it does, it will boot from the Vista DVD.