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Another way of doing it is:
WindowInteropHelper helper = new WindowInteropHelper(this);
this.StartupLocation = System.Windows.WindowStartupLocation.Manual;
this.Left = System.Windows.Forms.Screen.FromHandle(helper.Handle).Bounds.Left;
this.Top = System.Windows.Forms.Screen.FromHandle(helper.Handle).Bounds.Top;
this = your WPF Window...