It's not a Linux question - or even Ubuntu, but...
You have the Ubuntu CD in the CD drive, right? If you do, then apparently your computer boots from the hard drive first. You can change that.
With computer's make/model in mind, find the manual online in order to learn how to get into the computer's BIOS and how to select a different boot device at boot time. Then you can either change the boot order in the BIOS or choose it at boot time.
If you want to hack though it instead, insert the Ubuntu CD in the CD drive.
As the system first boots, you may see a "Boot Options" menu. Choose it.
Then ask the computer to boot from CD.
Failing that, you can try to glean how to open your BIOS the same way.
As the computer first starts, look for "Setup" or "BIOS" or "System" or any kind of clue to learn the BIOS function key. Or maybe try the function keys as it is booting:
F1, F2, F10, F12, DEL, or some other function key perhaps.
Once you are in the BIOS, navigate to the "boot order" section.
Place the CD/DVD at the top - or the first boot device.