I have an ASUS M3N78-VM motherboard. It runs Nvidia GEO force 8200 video and graphics. I have my monitor connected to the DVI port. I tried to connect my HDTV to my pc with a HDMI cable. When I plug in the cable I lose the DVI and the monitor isn't recognized any longer. The TV works just fine. I can't detect the monitor, it is as if it didn't exsist. When I unplug the HDMI the monitor is recognized and returns to working properly. So what's the deal??
When you go to Display Properties (Screen Resolution), does it show that the card detects to monitors? More than likely one is disabled. What you have to do is extend the displays under "Multiple Displays" in the display properties. If it only shows one monitor, try rebooting the computer while both tv and monitor is connected.
I'm guessing that your motherboard or graphics card doesn't support the viewing of two screens at once and that you may be encountering problems here. weird seeing as windows usually support a minimum of two monitors natively.
EDIT: I just saw that your graphics are inbuilt into the motherboard, I'm assuming that since you have onboard graphics that this may be the cause. Your motherboard may not support multiple in/outputs with its inbuilt card, you may have to go discrete..