How old and can it boot from a drive?
That is to say, is the problem that you have downloaded the live-cd of Ubuntu 8.10 and burned it at a low speed on a good quality CD as an ISO image file and it will not boot?
With the CD in the drive start the system, and at the start of the POST, look for information on which key to press for a on-time boot menu or for the BIOS setup. If you have a boot menu, choose to boot from the CD/DVD drive.
If you must enter setup, look in the BIOS for Boot Priority, and set the First Boot Device to the CD/DVD drive, and the Second to your HDD. This has the system look to the drive first for an operative system.
There is a Media Check function on the menu of choices when the CD boots that you might use to verify its quality.
It is good to try, but if your system is really old, with limited resources, Ubuntu not may be the best choice for you.
If I am answering the wrong question, it is because your query lacks details about the system you are trying to run Ubuntu on and what you have done. See this.