there is no script you have to change. your COMPUTER has to boot from cd. this has to be changed in the BIOS. This is on-chip in your computer. when you boot, you'll see a message like "hit the DEL key to enter setup". It might also be F1, or another key - but this "setup" is the bios. there you can find something like "bootupsequence", in my pc it is "first boot device =" "second boot device =" etc.

good luck!