If you have a Hardy or Intrepid installation and have downloaded the iso of Ubuntu, you may install Ubuntu from a USB pen drive if your BIOS supports booting from USB HDD.
Create a Ubuntu Startup Disk (you’ll find that option in the system menu) and then select the iso you downloaded. Then reboot and configure your BIOS to boot from USB HDD. If you are successful booting from it, you can install it.
However, if you get an error saying “Cannot unmount /media/disk” after you click install, you can mount any other drive and go through the process again, and it should work.
