Window and fulscreen differences?

Just a quick question, i have a mesh rendered in a windown and set the depth buffer for the renderer windowed to DX248 which looks fine, the only problem is, is when i go full screen the back of the mesh displays through the front. I expect it is relatively simple but it has me stumped!
“Oh and this is my first xfile export from blender :-)”

Se attached file for what im doing wrong?

DancingSnowman.rar (142.8 kB)

looks like you didn’t set the fullscreen depthbuffer format

…mmmm…

Please feel free to send me a virtual slap for that schoolboy error! I wont forget that again!

cheers