You need an up to date installation .iso, preferably the Dell Windows 7 Pro Skylake .iso see here for details: [URL]  Alternatively you can go through the procedure to manually slipstream, USB 3.0 drivers, SATA preinstallation drivers, NVMe hotfixes, the convenience rollup and IE11: [URL] 
Hello guys I desperatly need your help. I'm tryng to downgrade some workstation XPS 8910 and Aurora R5 from Windows 10 to Windows 7. Unfortunaly we can't use yet Windows 10 and i have a lot of problem installing both system. i tried to install with usb stick. The First problem i met was that after  installation start i don't have the use of mouse and keybords so basically i was not able to start the installation. i fix this problem serching on google and i have added the usb 3.0 driver with some utility downloaded from  intel  onto  WIM files of windows. So now i'm able to start the installation, i'm not able to finish In both workstation i have disabled secure boot and i enable legacy boot Into the Alienware Aurora R5 After the installation has finished during the first access where i have to create an username and assign the name of the workstation i l'm not able again to use mouse and Keyboard....so i can not do anything again. I thought that when i had inject usb 3.0 driver with the intel utility i had solve the problem....what i can do? maybe the utility i used have added the usb 3.0 driver into the boot.wim file only and not into the install.wim file? Into the XPS 8910  I have the use of the mouse and the keyboards at the start of the installation, but is impossible to start the installion because of the error " Setup was unable to create a new system partition or locate an existing system partition" I tried to delete all the partition during the installation of windows, i tried to clean the disk with the diskpart  utility, i tried also some tricks found on google..... but nothing.... I don't know what i can do......in both systems.... :-( Please Help      
Install Windows 7 on Dell XPS 8910 and Aurora R5