Basically you will need to install a bootloader (like GRUB) on your system. Part of this will be installed in the MBR (master boot record) of the startup disk, and the other part is usually on a separate partition called /boot (doesn't need to be a separate partition, though). I guess that your Ubuntu HDD has the boot partition already. You will need to install GRUB by using a Live CD or install CD to do so. The exact method depends on the tools at hand. Read the GRUB man pages and tutorial for the proper way.