The point is, the BIOS controls the boot sequence, i.e. which drive gets checked for a bootable system first. What you had before is this: CD first, then hard drive. Every time you booted up, your PC checked the CD drive, found nothing, then went on to the hard drive automatically.
I can't imagine why installing Ubuntu would change that, but to be sure: When you boot up, it usually says "Press (Esc/F2/F8/whatever) to enter setup". Do that to access your BIOS settings (usually blue page with lots of menus). Navigate to the boot settings menu and check if the boot sequence is alright, then save and exit. As suggested, one workaround would be to place the HD on the first place, but then you can't boot from CD until you change that back.
This might be a shot in the blue, but did you install grub to the Master Boot Record or to a local partition on your hard drive?