I have found the easiest way to make a bootable USB is to use SARDU. It is a utility that has buttons to download any of the files you might want on the USB then all you do is click the "Make Bootable USB" button. It is best to use a blank USB for this but not necessary. See my article on using SARDU:
http://www.experts-exchange.com/articles/Storage/Misc/Boot-Disks-UBCD-UBCD4Win-and-SARDU.html
