lines on screen in media centre

[arfur]

I've noticed while streaming films from my Vista PC to my Xbox using media centre that there are vertical lines down the screen, almost as if the screen has been split into blinds. As soon as the Media centre logo appears it seems to happen, the circle vista icon actually isnt a circle.

I'm using HDMI cables on a 1080I TV,

any ideas?

 

[Nismo]

Does switching to 720p do anything?