I agree with the other users here. Hard drives and CD-ROms (all flavors) should be on different channels whenever possible.
Primary Master : C: drive (HDD)
Primary Slave: 2nd HDD (if exists
Secondary Master: CD/CD-RW/DVD
Secondary Slave: CD/CD-RW/DVD
In my personal setup, I have an 18GB drive as C:, a 12GB drive as D:, a CD-RW as E:, and a DVD-Rom as F:
Since you have a CD and a CD-RW, I would make the existing CD drive the Master and the added CDRW as the slave. Just my personal prefs, though. Before I got a DVD to replace my CD, I set it up that way and used my CD for almost everything except when I was burning a disk. From a coist factor, I figured a CD drive failing from usage would be better than the CDRW failing from usage.