weird issue with communication between notebook PC and HDTV

I've been using my HP Pavilion dm1 notebook very successfully to drive a Samsung HDTV but we decided to move things around and I'm trying and failing to get it now to drive our Sony Bravia KDL 32-W5810 through the HDMI port. An image does appear on the TV screen but it's of the wallpaper only. Nothing active comes onto the TV screen although the laptop sreen is functioning perfectly. The wallpaper image aldo occupies only about half of the TV screen. Sound comes through the HDMI cable fine.

I'm not particularly technically minded but this makes no sense to me! Can anyone suggest what the problem might be please?

Thanks in advance