you probably are trying to boot into a CD but your BIOS is not configured for it.
at startup, look for "Press XX for SETUP" or, for a newer motherboard, "Press XX for BOOT MENU", something similar.
If you get into the boot menu, put the CD in the drive and select to boot from the cd drive.
If you can only go into setup, look for a "boot" menu and you can put the cd drive before the hard drive. They're all different so I can't give you any specific screen, etc.
once you tell it to check for the CD before going to the hard drive, it won't care what's on the hard drive and you can then boot from the CD