Create a Bootable USB Drive
1 Your flash drive will need to be at least 4 GB in size in order to successfully copy the ISO file onto it.
All of the data on your flash drive will be lost when you turn it into an installation drive, so be sure to back up any important files before continuing.
2 Download the Windows 7 USB/DVD Download Tool. This is available for free from Microsoft. Despite its name, this tool works with Windows 8 and Vista ISO files as well. You can install and run this tool on virtually any version of Windows.
3 Select the Source file. This is the ISO that you created or downloaded in the first section. Click Next.
4 Select USB device. Click the USB Device option. Select your flash drive from the list of available devices.
5 The program will format the USB drive to boot correctly, and then copy the ISO file onto the drive.
6 Boot it from the USB Drive