No picture output from any video files with plasma screen

[aww999]

My computer works fine when connected to my Dell monitor. When I just took it downstairs and hooked it up to my pioneer 507XD plasma screen, Windows worked perfectly, all the desktop came up as normal, but trying to play back any video files gives me sound, a moving scroll bar as time elapses, but a black screen. I am using Windows XP, media player 10 (also tried VLC, quicktime, realplayer, and media player classic), an HDMI output from my nvidea6800 video card into the plasma.

I tried using irfanview as a player, this gives me picture, but no sound! Bloody computers!

Any ideas what it could be?