This is a repost of a post from an old blog, made on August 19, 2012, that used to be on:
http://beginner.adminramble.com/install-windows-7-windows-8-usb-stick/
Original post:
Here is a small guide how to make a bootable USB pen drive, from a ISO image of Windows setup disk, which you can use to install Windows 7 or Windows 8 on your computer that doesn’t have a DVD drive. The USB stick that is going to be used needs to be at least 4 GB and all the data on it will be erased in the process.
First thing you going to need is Windows 7 USB/DVD download tool that can be downloaded from here. Download the file and install it on your computer.
Run the program and point it to the location of your ISO file, I used the Windows 8 RTM evaluation ISO here.
(If you have a .img file you can just rename the .img extension to .iso, or use a converter like ISOBuddy or MagicISO to convert from some other image format to ISO file)
On step 2 choose USB device as the media type.
In step 3 select the drive in which your USB stick is plugged in, press Begin copying and wait for the process to finish.
The tool will reformat your USB stick and put Windows setup files on it.
After it is done you can unplug your stick and install windows on any computer that can boot from a USB ( if a computer is not starting the Windows setup, check your BIOS if it is set to boot from USB before the hard drive).
If you get “We were unable to copy your files. Please check your USB device and the selected ISO file and try again.” error check this post
Besides Windows 7 USB/Download tool you can also try to use Universal USB Installer
