I do this with my TV too
The first place I would look is to make sure that Windows detected the HDMI display properly.
Look in Device manager with the TV connected and turned on.
Click the Windows Orb (used to be the START button in XP)
and you should see a link to click for it.
In the Device Manager window see if there is anything under
or if any items have a question mark or exclamation mark on them.
If you see anything like that let us know what it is and someone can tell you what to do next.(it depends on what exactly you see)
Also what kind of video card are you plugging the HDMI into? I use an ATI card.