If you have a iso image of the installation of any OS, you need not write the image onto a cd to do the installation, here is way to easily turn your external hard disk or your pendrive into a bootable install device.
1. Plug in the usb device
2. In you ubuntu go to system->administation->startup Disk Creator.
3. This will open up a window with two fields
Source Disc image(.iso) or CD
Using the button "other" just below this option point to the .iso file that you have.
4. The second option is the "Disk to use". By default your pendrive or the hard disk will be shown there, just click on the write one and click on "Make start up disk"
Once the above steps are done successfully, you can plug your pendrive or harddisk to any system that supports booting from external usb devices and run the installation.
The same steps can also be used to copy an installation "cd" to a pen drive and make the pen derive work as the installation media. For this you just have to put your "cd" into the "cd" drive and in the first step point to the cd drive.