1. Go to this Microsoft site "Upgrade Windows with only a product key" and download the Windows 8 (not 8.1) installer. (It's small).

  2. When the download is complete, launch the Windows 8 installer and enter a legimate retail Windows 8 product key when prompted to do so.

  3. When the download of Windows 8 begins, wait until it reaches 1%, click the [Pause] button and then e[X]it the installer. (Do not simply quit the installer).

  4. Back on the same Microsoft site, (in Step 1 above), download the Windows 8.1 installer this time. (It's also small).

  5. When the download is complete, launch the Windows 8.1 installer and you'll see a screen telling you that the "Download did not complete successfully" along with an "Element not found" error. This is normal. E[X]it the installer.

  6. Re-launch the same Windows 8.1 installer and this time the Windows 8.1 ISO will start downloading.

  7. When the download is complete, select "Install by creating media" > click [Next] > select "ISO file" > click [Save] > select a location on your hard drive > give it a filename (such as "Windows_8.1.ISO") > click [Save] then [Finish].