Your onboard is usually disabled at bios when a dedicated gpu is detected. Some motherboard give the option of allowing you to run both at the same time. Tipically it'll be Onboard GPU:
-OFF
-ON
-Off when Dedicated gpu detected
Pick on and once into windows it should auto pick it up and install it. Plug your dvi/vga into the onboard right click>propertise (On desktop)>settings double click the disabled monitor set resolution and ok.
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