One of the main reason why i love Windows 7 is to skip the product key input during the installation. I believe there are reasons why Microsoft had to put a stop son this but there are also valid situations where skipping a product key during the installation also makes sense.
To achieve this , we need to edit the ei.cfg(edition configuration) file present inside the/sources folder of the ISO image. All we need to do is to provide the version details inside this file so that Windows Setup is not needed to ask for a product key to install the appropriate version relevant to that product key.
1.If you are going to install Windows 8.1 using a USB drive, transfer the installation files to the USB and then proceed to step 2. if you need to create a DVD, open the ISO using a utility such as MagicISO so that the structure is not affected.
2.Browse to the /sources folder.
3.Look for the ei.cfgfile and open it in a text editor such as Notepad or Notepad++ (preferred). If the file doesn’t already exist, create a new text document and rename it as ei.cfg
4.Now amend the file depending on the version which is included in your particular copy and the version which you want to install. For Professional edition which will be the best bet for most of the users, paste the following context into theei.cfgfile. Make sure that anything that already exists in the file should be cleared first.
5.Save the file and run the Windows 8.1 setup again. The setup will not ask for the Product Key anymore.
This is not a hack or anything illegal. The configuration above is supported by Windows installer and is well Documented by Microsoft and can also be used on some other versions of Windows as well.
Credit to www.techmesto.com