In order to figure out what is causing your Grub error 21, I think it would be helpful to get some more info about your setup. How about booting your Live CD (the Ubuntu install CD), download the Boot Info Script
to your Ubuntu desktop, open a terminal (Applications > Accessories > Terminal) and do:
sudo bash ~/Desktop/boot_info_script*.sh
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 that file to your next post, highlight the copied text, and click the pound sign # graphic in the 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 your booting problem might be.