matilda, many machines can boot from CD. During start up check, if poss, the dialogue that scrolls up the screen (usually quite quickly) for a line referring to an 'Function' key, usually F7 - F11.
When pressed during start up, your machine will enter a boot menu, listing all available options, from which you select where to boot from.
If your CD drive is featured in that list, select the option and away you go.