If you've downloaded the Windows 8.1 ISO and need to perform a clean installation, you can activate it using a Windows 8 product key. The following instructions have been shamelessly taken from Microsoft and the "Windows Valley" website.
The prerequisites for this work are:
- A Windows 8.1 ISO (or USB flash drive with an extracted ISO on it)
- The Windows 7 USB/DVD tool (if you only have the ISO file for Windows)
- A valid Windows 8 product key
Once you have the ISO, you need to extract its contents to a USB flash drive using the Windows 7 USB/DVD tool. Don't worry, it works for Windows 8 (and Windows 2012 as well).
Next, you need to follow the instructions provided by Windows Valley and create a file called "EI.CFG" in the Sources folder on your USB flash drive.
For the plain vanilla Windows 8.1 Professional, the file needs to contain the following text:
[EditionID]
Professional
[Channel]
Retail
[VL]
1
If your edition of Windows is not Windows 8.1 Professional (i.e. the N edition or the Windows Media Center Edition), refer to the Windows Valley link for what value to use for "EditionID".
Save this file (make sure you save it as EI.CFG not EI.CFG.TXT!) and then safely eject your USB flash drive.
You will now be able to boot off this drive and install Windows 8.1 without a product key. Once you have installed Windows, the only way to then use a Windows 8 key is via the command line; you cannot enter a Windows 8 Product Key using the product key GUI as it will reject the key.
To get around this issue, open an Administrator command prompt, and then use the following command:
Cscript.exe %windir%\system32\slmgr.vbs /ipk <Your product key>
That's it, you're done!
