Problem on Camera

I've got a problem on CFirstPersonCamera . My desktop screen resolution is 1024x768 and if I create a application using 800x600 in window mode,there is no problem about camera.But if the applicaiton use 800x600 in fullscreen mode,there seems to be some deviations. (1024 x 768 in fullscreen mode is OK).I think there might be something about camera I do not know. Here is my code: //When app initialize D3DXVECTOR3 Eye( 0, 10, -30.0f); D3DXVECTOR3 At( 0, 0, 0); m_Camera.SetViewParams( &Eye, &At ); m_Camera.SetProjParams( D3DX_PI/4, 800.0f/600.0f, 0.1f, 1000.0f ); //when app on Reset Device m_Camera.SetProjParams( D3DX_PI/4, 800.0f/600.0f, 0.1f, 1000.0f );