a few things for you to consider
you say that another cd drive works in this system?
is this a dell cd drive? if not, the bios may not have the right driver for it
try reseating the ide and power cables to the cd drive
check the jumper settings, then clear nvram (go to system setup, make sure that caps lock, num lock, and scroll lock are all lit up, then hold alt, and tap F, then tap E, then tap B and the system should reboot) if that does not work, try popping out the cmos battery for a minute or so
try this cd drive in another system and see if it recognizes it
swap ide cables
it may be a bad cd drive.
if it is a good cd drive, and you get it to recognize it but can not get it to boot from it, go to support.dell.com and download the file CDENAB.EXE then, go through the instructions to make a floppy disk, use the floppy disk to make the cd drive bootable
hope some of that helps