Once again, I don't really advice using a rescue disc in this instance as it's overkill. But to answer your question, in order to get your computer to boot to the CD, you need to go into your system BIOS, and set the boot order to your CD as the first device. Save the changes, and exit.
Getting into your computer's BIOS screen is different depending on your system. Some by holding down the DEL key upon startup. Others use a specific function key. For instance, my Acer Netbook uses the F2 key to enter the BOIS. And others will even tell you which function key to use for selecting the boot device upon startup. Look at the DOS like text when powering up your system to see if indicates a specific function key to use?