I have tested and used Andy's suggested method before and I will vouch for it as an advanced solution to your problem. In my experiences, I found that the CD version of PE seemed to load faster, but once in the PE environment everything was the same no matter which media you use to boot the system.
For someone who is unfamiliar with Windows Vista deployment technologies, it is an easier option to purchase a USB DVD player. Also, you will probably want one in the future to install software.
I would look to the manufacturer first to see if they have a recommended external drive, but if they do not
here are some alternatives.