|There are two different Pavillion 8670C models, and probably two different mboards.|
If yours is D9290AR, it probably has two sets of IDE controllers - a regular one, and a RAID one, or it has a regular IDE controller and a RAID controller that supports both IDE and SATA, and 4, or 3, IDE connectors . Not all RAID controller chipsets support ATAPI, which is required for optical drives (e.g. many Promise ones don't).
If you have the CD drive connected to the RAID controller, connect it to the other one instead.
Alternately, if this large hard drive is the only one on the computer and it is empty, or in any case if you are trying to install Windows on the large hard drive, if you have the CD drive connected to the RAID controller, Windows probably doesn't have the built in ability to recognize it and provide drivers for it even if the RAID controller does recognize ATAPI drives, and you must load the drivers for the RAID controller at the beginning of Windows Setup for the RAID controller (press F6 when you are prompted to load drivers for SCSI drive controllers or similar, then provide a floppy with the drivers for the RAID controller). If you don't want to have to do that, connect the drive to the other IDE controller.