According to Ubuntu official website, installing Ubuntu 8.04 or upgrade to Ubuntu 8.04 from older version need CD. But in fact, burning a CD is not necessary. If you have installed a distribution of Linux but have no CD writer, you can also install Ubuntu 8.04 by the ISO file. I installed it from hard disk just now and now I am going to write something about how to do it.
First, make sure that you have more than one partitions (except swap partition). For example, I have two partitions /dev/sda1 (mounted as /) and /dev/sda5 (mounted as /home). Since I planned to install Ubuntu 8.04 on /dev/sda1. I downloaded the desktop iso file from here and save it in /home, the /dev/sda5 partition. So the full path of the ISO file is: /home/ubuntu-8.04.1-desktop-i386.iso.
Second, extract /casper/vmlinuz and /casper/initrd.gz from the ISO file. Since I had installed Ubuntu 7.10, I need to execute the following commands:
sudo mount -o loop ubuntu-8.04.1-desktop-i386.iso /media/cdrom0
sudo cp /media/cdrom/casper/vmlinuz .
sudo cp /media/cdrom/casper/initrd.gz .
Now edit the grub configuration file /boot/grub/menu.lst. Add this item:
title Install Ubuntu
kernel /vmlinuz boot=casper iso-scan/filename=/ubuntu-8.04.1-desktop-i386.iso
Note: the red text (hd0,4) means the fifth partition on the first hard disk. i.e. it means /dev/sda5. The partition number of grub is from zero, not one.
Save menu.lst and reboot your computer. You can start Ubuntu 8.04 live CD from grub and install it to your hard disk as using CD. The only thing you must care about is that you mustn't format the partition which contains the ISO file. In my installation, the ISO file is at /sda5 so I can't format it. In addition, I mounted /home onto /dev/sda5.