look in the bios settings for the boot priority, set your CD to boot first and the HDD second.
the bios settings are reached by holding a key down while starting the computer.
This key varies according to model. Look in the user manual or download the manual from dell, to locate the directions for bios settings.
Yes, check that , but you might want to check if there isn't something related to your IDE / SATA in bios as well. You might try change that too!
Good luck!