What I would recommend you is not to focus an element explicitly, but to put it in the first place in Xaml in the Window's content or to use the TabIndex properties of the visual elements. As I noticed that your window is called LoginWindow I guess it is opened at the start of the application. Please, notice that the Silverlight plugin must be focused in order your TextBox to be focused.
The work-around if you still want to do that with code is to focus the element asynchronously with a Dispatcher.
the Telerik team