The easiest way to turn a USB flash drive into a bootable Windows 7 installer is by using the tool Microsoft offers, cunningly named the Windows 7 USB/DVD Download Tool. To get started, download the installer [exe] from Microsoft.com and follow the basic steps to put it onto your computer; you can put it on the computer you plan to install Windows 7 on or another one, it doesn’t matter.

Other method here: http://www.iamthefrenchblog.com/2013/05/installer-windows-depuis-une-clef-usb/