1) Download Windows 7 USB/DVD Download Tool from this LINK and install it .
DOWNLOAD
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2) Launch the Windows 7 USB DVD Download Tool. Click on Browse button and double click to choose Windows 8 ISO installation file.
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3)Click on Next and then Click USB device , select your USB key drive and click on Begin Copying
Click on Erase USB device and click Yes
5) Wait for the Windows 7 USB/DVD Download Tool to go through the procedure.
And that is all!