You mean you can't enter the BIOS? Hit the Delete key or F1 (whatever key it says "to press to enter setup"). Then adjust the boot priority so that CD-ROM is higher than hard drive.

If that doesn't work (you can't see the CD drive in the boot sequence), maybe your BIOS doesn't support CD-ROM booting (older BIOSs). You must then use a floppy to boot the computer (or the hard drive).

Aye, the motherboard must be configured to have a boot device order where the CD/DVD drive is first, then the hard drive proceeding.

Some motherboards have a boot menu on a key, like my ASUS, where you can call up a menu on which device to boot from.