You're saying it was rebuilt?
First thing I'd do is open it up and make sure the power and data cables are firmly seated. I'd also check to make sure the jumpers are set correctly. There are thousands of posts and guides online that show how to check if the little tiny jumper on the back of the drive is set correctly for its position on the IDE cable.
Also try "uninstalling" the device in Device Manager, then reboot. Windows should see the devices and install drivers. The drivers are very generic for ATAPI optical drives.
Another very simple thing to do is disconnect the optical drive and trade with someone else. If their drive works in your PC and yours doesn't in theirs, then the folks who rebuilt your PC dropped yours on the ground or swapped it for a bad one.
If an optical drive worked in Win 98 it certainly should work in W2K.