In order to get a clearer picture of your setup, I suggest to boot from the Ubuntu LiveCD and download the Boot Info Script to your desktop:
Then open a terminal (Applications > Accessories > Terminal) and type:
That will create a "RESULTS.txt" file in the same directory from where the script is run, namely your desktop; please copy/paste the contents of the RESULTS.txt file to your next post, highlight the copied text, and click the pound/hash sign "#" graphic button in the Ubuntu forum message box so that the text will get "code" tags put around it. The results of that script will help clarify your setup and hopefully what the solution to your booting problem might be.
sudo bash ~/Desktop/boot_info_script*.sh