SergRed, please let me know the following information:
I turn on Legacy Boot and Sleep Mode appeared in the shut down menu. However, the built-in video card does not work at all, although in the Device Manager in working order. Previously PC was booting in UEIFI mode only.
Since you mention that this video card was working fine on a Windows 8 64 bit environment, this could be a compatibility issue with the current operating system, w8.1 and the video card drivers. So I recommend you get in touch with Asus.