You should be able to enable and disable onboard video in the BIOS. You can access the BIOS by either pressing F2, F8, F10, F12 or DEL during the computer's post.

If onboard video is determined through jumpers, the only help you're probably only going to be able to find is through HP or an HP Support Forum. Manufacturers use proprietary motherboards that are mass-produced for certain models of their computers. So even if Azza makes boards for the average consumer, your PT-994BX will most likely be different than one purchased from a retail outlet.