This is one common solution to this issue, please let us know if it works for you:
"When Windows is asking for driver, just click Cancel. You will be brought back to the welcome screen. At the welcome screen, remove your USB drive, insert it back to DIFFERENT USB PORT. Click Install Now again. The installation process will be like usual.
This problem happened to all new version of updated Windows 7 installer. I think, it is due to failure of Windows installer to remain its detection to the USB drive. It lost the connection, and became confused, don't know where to find the USB drive it used to read before.
When we re-insert the USB drive, Windows installer will detect the USB drive back, and continue like usual."