First of all make sure your not sharing IDE connections.
Put the hard drive in IDE 1, and the DVD drive in a completley seperate IDE 2 (there is definietly two), using a seperate ribbon connector, and not just linking them on the same.
If this doesn't work or you've already got it like that, you'll find that your DVD rom drive is set above your hard drive in the loading sequence, or worse, not at all.
In this case, you'll need to go into the BIOS.
To do this, press Del when the computer is starting up (check instructions on bootup to what the key actually is, though it is usually the delete key on most computer setups).
It'll then appear, and in one of the main menus (first two I believe depending on BIOS make) you'll find this type of sequence:
1st boot device
2nd boot device, etc
Set the first to the hard drive, IDE slot (Making it see it first, before anything else), and set the second to the DVD drive.
If you have a Floppy Disk drive, I'd set this as the 3rd bootup device.
Then press Escape to navigate back through the menus, and select to Save and Exit. This will save any changes, and your computer should boot up!
I hope this helps
If you wanted to install Windows or something, that needed a bootup of the DVD drive/Floppy drive before the hard drive loads, just go back to BIOS, and change the boot order, and resave.
Then just place it back once your done