While trying to reinstall Vista, the computer wont recognize the boot disk?

while working on my friends computer, he asked me to uninstall Vista and replace it with XP. I deleted the partition and replaced it with XP. midway through setup, my friend informed me that the XP disk had already been used on another computer and therefore his product key was invalid. I got his old Vista disks and began the process of replacing the XP with Vista. However, when booting from the CD drive, the following message appears: No boot device detected. Could it be that the Vista boot disk is a DVD? If not is there any other solution to the problem? The computer is a Dell Dimension E521. Thanks.