I can think of three likely problems and one unlikely problem.
1. Did you really burn the ISO image to the pen drive? Did you use software such as freeisoburner: http://www.freeisoburner.com/
to burn the ISO to the drive or did you just copy the ISO to the drive? If you just copied it, then the computer can't read the files
2. The USB drive may be broken. Have you tried burning the ISO to another flash drive or disk?
3. The copy of windows you downloaded wasn't the whole thing and there are missing files that once again, the computer can't read or find
4. Your USB ports on the computer is broken.