that's one way - to find a IDE cable which can hook up two IDE devices, and set your HD as the master, set your DVD-ROM as the slave.
solution 2 - to get a 5.25 inch enclosure for exteral drive, and turn your DVD-ROM to an exteral device.
solution 3 - to get an IDE to SATA convertor, and connect your IDE HD to SATA port on motherboard.