Downgrade rights for the Dell Windows 8/8.1 Professional OEM license require a Dell Windows 7 Professional Reinstallation DVD in order to use OEM System Locked Preinstallation. See here where I explain the licensing and how SLP activation works in more detail: [URL] OEM licensing is a minefield and with Microsoft breaking the Digital distribution of windows 7 its becoming even harder.  From what I've gathered it seems that you are trying to exhibit downgrade rights by installing from a retail Windows 7 installation DVD which is not supported by Dell. If you only have a retail DVD, follow the instructions to convert it into a bootable USB for installation with the GPT partition scheme on a UEFI BIOS: [URL] Install without a product key and apply Dell OEM SLP using the Activation Backup and Recovery program and Dell cert files as instructed. If you have a Dell Windows 7 Professional Reinstallation DVD it is better to convert that to the Bootable USB as the Dell OEM SLP will be automatically applied without the use of the ABR program or Dell cert files. 
XPS13 (early 15) purchased for CFO.  No 7 Pro configuration available so I purchased an 8 Pro model with the intention of exercising my downgrade rights. Spent 4 hours shoehorning 7 Pro plus all the drivers onto it.  Last step is activation.  Try to activate and call Microsoft per their fact sheet:  [URL]#fbid=Ybp3jT2oAFN  They tell me to call the OEM as only the OEM can issue OEM keys.  Call Dell (politely and patiently) over and over again and get: -hung up on -told to call Microsoft -told to return the system and order a Windows 7 version (doesn't exist) -told to order Windows 7 media (doesn't exist) -told repeatedly by multiple reps that Dell has a "special" OEM license that does not include downgrade rights.  How on earth do Dell expect to service the SMB space (i.e. folks not volume licensing their OS) with this runaround and ***-and-bull story?  Does anyone know of the correct channel at Dell to get this very basic need met?  
DELL refuses to honor Windows 8 Pro downgrade rights