Halo Custo Edition only launches in windowed mode.

It’s a pretty bizarre situation. Whenever I try to launch Custom Edition, it will give me the error message “A problem occurred initializing Direct3D. Hardware acceleration maybe disabled, please run DXDIAG.” I know that’s a load of baloney because everything comes up green on dxdiag. However, if I play the game in windowed mode, it works perfectly. Does anyone know a solution, or can refer me to a place where I might find one?