Source: lifehacker.com and winsupersite.com
If you do a clean install using the Windows 8 Upgrade Assistant, you should be fine—but if you've already formatted your drive or you're moving to a new drive, you can't do a "clean install" without installing an old version of Windows first. It'll let you install Windows 8 cleanly, but when you go to activate, you get an error 0x8007007B, saying your product key can only be used for upgrading.
If you get that error, here's how to fix it:P
regedit. Press enter to open the Registry Editor.HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE/and double-click on the MediabootInstall key in the right pane.cmd. Right-click on the Command Prompt icon and run it as an administrator.slmgr /rearm and press Enter.When you get back into Windows, you should be able to run the Activation utility and activate Windows as normal, without getting an error.
Getting the Windows 8.1 ISO is quite easy using this very simple method.
In a web browser, navigate to Microsoft's Upgrade Windows with only a product key page. Then, click the light blue Install Windows 8 button. Windows 8 Setup will run. Input your Windows 8 product key when prompted and Setup will start downloading. After the download begins, close Setup.
Now, return to that Upgrade Windows with only a product key. This time, click the light blue Install Windows 8.1 button. Windows Setup will run, as before, but you won't be prompted for your product key. Instead, the Setup ISO will download automatically!
After the download completes, Setup will say that it is "getting files ready" (uncompressing the installer) and then present you with the Install Windows 8.1 phase of the wizard. Choose Install by creating media and then click Next. In this next step, choose USB flash drive or ISO file.
Once you've downloaded that ISO, you still can't use your Windows 8 product key to perform a clean install of Windows 8.1. You can, of course, use it to perform an upgrade from Windows 8.
When clean installing Windows 8.1, your Windows 8 key will not be accepted at the setup screen. To bypass this use on of the following generic keys:
When you are finished installing, open up the Explorer and select System Properties. Select change product key and enter your actual Windows 8 product key, You should now be able to activate Windows and have a clean install of Windows 8.1 using official media.
The only difference with the above method is that you have to remove all the installed Product Keys used. To do that right click on the bottom left corner (start button) and select "Command Prompt (Admin)". Type slmgr.vbr /upk in the command prompt and hit enter. Restart and enter your new Key.