This happened to me on my desktop whith a Asus P8Z68-V motherboard.
I have 3 monitors and so I needed a second VGA for the two secondary monitors.
I had an old Geforce 8400GS laying around and so I decided to use it.
I wanted my main VGA to be the onboard Intel that amazingly has better performance. So I setted up on Bios as main VGA.
All went fine until MS did their number... Windows update changed the Graphics adapter driver.
I was unsuspected of that until I rebooted the pc. The log on screen was not to be seen but only a black flickering screen. So after some trial and error here is what I did.
I went to the bios and changed the main VGA to be the Nvidia. Booted normaly. I found out after many tries that Windows keeps getting the Nvidia driver from the internet regardless if I disabled that option in Windows Update.
So you need to Run "gpedit.msc" and go to: Computer Configuration - Administrative Templates - System - Internet Communication Management - Internet Communication Settings - "Turn off Windows Update Device Driver Searching" and set this to ENABLED.
Download the latest Nvidia driver, run the installer, let it extract the driver and then cancel it.
Delete all Nvidia folders on program files.
Go to safe mode, uninstall the device in device management and make sure to tick the option to delete the driver.
In the windows folder search for the usual Nvidia files such as nvdisp.ini, make a search for Nvidia and nv*.* and delete everything related to Nvidia.
On Device Manager, your Display adapter should appear as "Microsoft Display Adapter" instead of the WDDM drivers for your card model. If not you must retrace the steps.
Right click the "Microsoft Display Adapter" and hit "Update Driver Software". Select "Browse my computer for driver software", then "Let me pick from a list of device drivers on my computer" and then select the "Have Disc" option.
Point Windows to the file "nv_disp.inf" that will be in your Nvidia folder, mine was: C:\NVIDIA\DisplayDriver\327.23\Win8_WinVista_Win7_64\International\Display.Driver\nv_disp.inf
Reboot, set your Intel VGA has main adapter once more and pray for uncle Bill for it to work.
And that should be it.
Hope it helps someone lost on the interwebs with the same issue as I.