title: WPF: Error getting UserControl into ToolBox
body: <p>I can remember that some time ago when I created a UserControl in Visual Studio 2008 the USerControls automatically appeared in the ToolBox.</p>
  
  <p>Now they don't. If use "Choose Items.." on the ToolBox I get the error message "The given assembly name or codebase was invalid", after selecting my assembly.</p>
  
  <p>I have Windows XP SP3 and .NET Framework 3.5 SP1.</p>