Let's say your resolution is WxH. So you are creating a window with dimensions WxH and you are creating a backbuffer with size WxH. Then you are drawing the backbuffer to the client rect of the window. The catch is that the client area of the window is usually smaller than the window itself - the dimensions of the window also include the title bar and all the borders. This way you are trying to draw an image onto an area that can't contain it without distorting it first.
If that's the case you want to resize the window and make it larger so that it can contain client area that has the exact same size of the backbuffer. To do it you call the AdjustWindowRect system function.
Hope that helps
