Windows Media Center can't play videos

Hi,

I have a Core 2 Quad Q9650 processor running at stock 3.0 GHZ, 4GB RAM, a GTX295 and Vista Home Premium SP1, all drivers updated and my problem is that when I try to play ANY video in Windows Media Center (even the ones that Vista ships with), I get a black screen, but can hear the audio.

All other video players (Windows Media player, Real player, itunes etc.) on my PC play videos without a problem, so I doubt it is a general codec issue.

I remember being able to watch the introductory video (the one with the family in front of a tv where thingsl like contrast and brightness are explained) about half a month ago when I first configured Windows Media Center.

Can you please help?

Cheers