The problem might be that the computer wants to use the video card instead of the onboard. If the video card has two DVI outputs you can get an adapter for DVI to HDMI fairly cheap, and you wouldn't have to fiddle with the software as much.
Other things to check for is if you have downloaded the latest driver for the HDMI port:
http://www.biostar.com.tw/app/en-us/t-series/driver.php?S_ID=374
(HDMI is at the bottom)
I don't know about HDMI, but my WinXP computer won't recognise my S-video unless it's plugged in during boot.