Turn off NVIDIA on Windows 8.1 Bootcamp
If you are trying to disable the nVidia graphics card, you can do the following:


Reboot into safe mode. If you don't know how to, refer to here for more information. Make sure that the Mac is configured to boot from Windows in the Boot Camp Control Panel.
After booting into safe mode, open Device Manager by pressing Win+R, and then typing devmgmt.msc.
Expand Display Adapters
Right click on the nVidia driver and select Disable. MAKE SURE THAT YOU DO NOT DISABLE THE INTEL HD GRAPHICS OR IRIS PRO
Reboot the Mac into Windows 8.1
If it boots normally, great. If it screws up, do steps 1-3 and select enable.


Note that this will DISABLE your discrete graphics, so that means that you cannot use it until you re enable it in Device Manager.