there is a simple tool from Microsoft that will do all the process easily called “Windows 7 USB/DVD Download Tool”
you must first make sure you have already purchased the Windows 7 ISO download from Microsoft Store and have downloaded the Windows 7 ISO file to your hard drive.
INSTALLATION
TO INSTALL THE WINDOWS 7 USB/DVD DOWNLOAD TOOL:
- Click to open the Windows 7 USB/DVD Download tool installation file.
- When you are prompted to either save the file to disk or run it, choose Run.
- Follow the steps in the setup dialogs. You’ll have the option to specify where to install the Windows 7 USB/DVD Download Tool.
NOTE: You need to be an administrator on the computer you are installing the Windows 7 USB/DVD Download tool on. The tool requires the Microsoft .NET Framework version 2.0 or higher.
you can also find more about it on microsoft’s website :
http://wudt.codeplex.com/