This method is contributed by freeflyfish , I have tested it. It is really very easy way to install Ubuntu 8.04 from hard disk.
Step 1: Download below package and exstract it to C:\
Step 2: Rename Ubuntu 8.04 xxx.iso file to be ubuntu-8.04-desktop-i386.iso , and then copy this iso file to your C:\.
Step 3:You also need to extract \casper\initrd.rar and \casper\vmlinuz from the iso image to C:\
Reboot computer, done!