With Windows 7, you could skip entering a product key during setup. You would then get a 30 days grace period before you had to activate, which could be pushed back a couple of times. You could also delete the ei.cfg file from the sources folder and during setup choose which version to install.
Windows 8 comes in just two flavors, Core and Pro. (And Enterprise and RT.) The version you get depends on the product key you enter during setup, which you can no longer skip. But clever heads have found out that if you ADD the ei.cfg to the sources folder you can not only choose which version to install, but skip entering the product key.
Just copy the few lines below into notepad and save it as ei.cfg.
[EditionID] Pro [Channel] Retail [VL] 0
This will of course give you the Pro version.
If you’ve been smart and used the Windows 7 USB DVD Tool to turn the ISO into a bootable USB, just copy the file to the sources folder and you’re all set.