Have you made any hardware or software changes around the time the problem started occuring?
Do your BIOS and/or start-up messages show any mention of either drive?
If it's a hardware fault, you might be able to narrow down the culprit by doing the following (pay attention to Master/Slave jumper settings on the drives!):
1. Remove the DVD from the system; see if the CD drive is then at least recognized by the system. If not:
2. Remove the CD and install the DVD by itself.
3. Install each device (one at a time) on the other IDE channel.
Although I've never seen this happen so severely that the devices don't even appear in Device Manager, there is a possibly related software conflict caused by some CD/DVD burning programs (Roxio is one). Read more about it here:
For Win 2K: http://support.microsoft.com/default.aspx?scid=kb;en-us;Q270008
For XP: http://support.microsoft.com/default.aspx?scid=kb;en-us;Q314060
In the above articles, mention is made of changes to the HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\ branch of the Registry. Be aware that the changes may also have to be duplicated in the ControlSet001, ControlSet002, etc. branches as well.