You need to change your BIOS to boot the CD first instead of the hard drive. Most BIOSs can be accessed by hitting the delete or F1 key when the first screen comes up, usually right after the very first beep when you turn it on. I usually just keep hitting one or the other until I see the BIOS screen. Usually they have a blue background.
If you are not familiar with the BIOS settings, be careful. Some settings, especially speed stuff, can cause damage. You may want to go to the manufacturers website or search google for better info.
Hope that helps.