Just a suggestion >> if the drivers arent properly installed *first* then you wont be able to disable the existing GPU within windows and install nvidia drivers later. My htpcs have integrated graphics, everytime I chuck a graphics card in, I found the fiddling needs to be done later. I have found The WDDM drivers do not support multi GPU instances.
I'd think about
>Uninstall the drivers and device(nv card) from your device manager
>run a registry clean
>Reboot > check your integrated rivers are working >> when the "found new hardware" for the nv card comes up > dismiss it. If at this stage your integrated drivers werent installed, install them(reboot) otherwise go to next step >>
>>Run the latest driver installation for your nv card >> reboot.
Then do what Q says in step 2 above.
Let us know how you go