you are right phil22, the onboard gpu was disabled. turns out it was the onboard gpu utility software that automatically extended the desktop to the unknown monitor. thanks for replying!
i found a solution for installing my graphics card here. http://h30434.www3.hp.com/t5/Desktop-Hardware/6-Long-BIOS-beeps-after-video-card-install/m-p/2391415#M60219
basically, because my pc was on windows 8, bios legacy support was disabled by default. however, enabling it required disabling some boot security feature in the bios that was specific to windows 8 and other OS that support it. my graphics card isn't new but is still a little better than the onboard gpu and i am the only using my pc so i guess the tradeoff is acceptable.