MDX 1.1 transform coords from screen to 3d world

[AlexDR]

I'm trying to convert the low rigth corner of my full screen app from screen coords to 3d world coords.
This is my C# code:

Matrix mView = device.GetTransform(TransformType.View);
Matrix mProj = device.GetTransform(TransformType.Projection);
Matrix mWorld = device.GetTransform(TransformType.World);

Vector3 screencoords = new Vector3(device.viewport.width, device.viewport.height, 0f);

Vector3 coords3d = Vector3.Unproject(screencoords,device.Viewport, mProj, mView, mWorld);

but without result, because the 3d world coords returned are far away from the mentioned corner.

can anyone help me?
thanks.
Alex