This differs from PC to PC. When you boot up your computer, after POST (the black screen with the memory test etc shown before you see the bootloader), you should see a prompt or message that shows you how to access the BIOS settings configuration tool (usually you have to enter some sort of keystroke e.g. F2, DEL etc). Once you are in the BIOS settings, there is an option that you can use to change the boot order of your devices. Make sure that the cd/dvd option is set before the hard drive.
how can I configure the BIOS to boot from cd ???