Hi,
I had never had problems with it. I watch movies on my HD tv usig my Macbook 1.1 using miniDVI to VGA.
What Windows are you using ? What MAC are you using ?
Please try to install the latest video driver. In my case it is the intel video driver. It has a tray icon and what you can do is to force the screen to use the secondary screen on it. Click on the tray icon then you are going to see monitor or something like it. Select the external monitor. It should do the trick.
Another way would be to boot up Windows with the miniDVI and TV already connected.
I hope it helps