For those of us without an optical drive, life can be tough when you need to install something during boot time. My CD-ROM drive is broken, so I was able to install Ubuntu off of a bootable flash drive. With USB sticks being so cheap these days its a perfect replacement for a non-functional optical drive. In any case, to make your flash drive bootable I used a utility called HP Drive Key Boot Utility. It was developed by Hewlett Packard, and is very easy to use. The utility reformats your storage media and allows it to be bootable. I’ve tried it on both USB flash drives and external hard-drives. Take note that your BIOS needs to be able to support USB devices before you can boot from your newly formatted storage media.
More precise instructions are available at this site as well. However, running the wizard step-by-step did the trick for me.