Your bios is booting from Hard drive before the cd .
You can choose the boot by typing F8 keyboard button in the starting.
in the starting boot press delete button
you can change this in the bios boot .
you have to make it boot from cd before the hdd .
next time share the bios version .
if you are running linux (ubuntu,fedora,...) use this command
sudo dmidecode -t 0
if you are running windows type in command :
if you are running mac os
you will find it in :
'about this mac' => More Info
you will find the bios version
after you this google it & you will get how to change the boot