Booting from USB ext HD via a CD

Hi All, I have a situation in that my notebook won't boot from an external USB HardDisk, and it does not have a floppy drive. What I wish to do, is install linux (any flavour as long as I can use XFCE), on the external drive but boot up via CD. I don't want anything written to the MBR of the notebook internal harddrive. I'm sure someone has either done this, or knows how to. Any help would be appreciated. I've been "googling" it for a bit, but still haven't found a proper method. Thanks -- SynapseR