Vostro 200 PCI-e Problem

Hi, as stated I have a Vostro 200 Slim, I bought an Asus EAH3450 PCI-e low profile, card works ok on another computer, in the Vostro I don't get any video, if I change the VGA conector to the onboard video with the computer on there's a screen saying that I must disconnect that cable and connect it to the new card, which is what I already did, its like the onboard video keeps detecting the monitor even if it isn't plugged in anymore, any ideas??? TIA