What version are you on? It would help if you include it in your signature so that the help can be tailored to your version.
In 10, go to View - 3D View Mode - 3D Window Settings and make sure that Transparancy in Shading is set to OFF.
In 9, go to the screen that deals with the 3D window settings and do the same. I don't have 9 running right now and I forget exactly what the menus are called to give you a step-by-step.