I am running Debian on this Desktop computer, but I wanted to install Ubuntu with it. There is plenty of disk anyway.
Start downloading Ubuntu and save it on your disk.
Install grml-rescueboot, with my Debian was:
apt-get install grml-rescueboot
Then as root, copy the ISO image to /boot/grml/
cp ~/isos/ubuntu-image.iso /boot/grml/
Update Grub 2 (You are using grub 2 right?)
You have to be root to run that command.
Reboot your PC, and you will a new option in your booting list. You can now install Ubuntu from your hard disk.