Do you get to any point where you see a welcome screen before it says "missing cd/dvd drivers"?
If so, retry again. But when you see the missing cd/dvd drivers hit cancel, and x out to the welcome screen. Unplug the usb drive and replug the usb drive into a different port. Then continue with the install.
Also make sure you use the Microsoft usb/dvd download tool. And check the hash. http://www.sevenforums.com/tutorials/31541-windows-7-usb-dvd-download-tool.html?filter=General%20Tips