Like gster said, reverse the normals. You might want to make only half a cube (delete faces you don't render) so your camera won't have to be inside it all the time.
BTW, tmujuru and GecT, to post full quality work you can just use image hostiing. I use imageshack.us and it works fine with 1.5MB size restriction wich I never reach.
It has some scaling problems with png format, jpeg works fine).
I feel like I'm advertising it too much. They're going to have to start paying me soon
When in doubt, delete history and freeze transformations.
My latest snake game
(Requires DirectX9 SDK and Framework)