It sounds like your CD/DVD drive is not selected as a bootable drive in your BIOS settings. I don't know how your bios is setup exactly, but there should be something like BOOT SEQUENCE or BOOT DEVICE PRIORITY or something similar to that. Usually it would go floppy (if you actually have one), CD/DVD drive(s), and then hard drives, and some now have flash memory drives as a selectable bootable drive. You need to make sure that you CD/DVD drive is selected somewhere in one of those settings.

If all of that is setup correctly the I would check to make sure that the disc is actually bootable. If the settings are right and the disc is bootable then the harddrive might need to have a full format, as oppoesed to a quick format, to install vista. I say this because I had a problem with my harddisk not being formatted first before trying to install.

If none of that works then I can't imagine where the problem would actually be. Hope this helps and solves your problem.