Booting from a CD has nothing to do with Grub. If the CD drive is enabled as a boot device (before the HD) in BIOS and the CD in it is a bootable one, the OS present on CD shouldn't allow Grub to load, but load itself instead.
As for booting an OS on another partition, add an entry into /boot/grub/menu.lst which should look like this:
as an example for a Windows OS.
For more information, check http://www.gnu.org/software/grub/man...tml.gz#Booting