How to completely reset the Toolbox and enable Visual Studio to rebuild it from scratch
http://www.devexpress.com/Support/Center/p/K18560.aspx
After installing or updating our products, you may experience issues with Toolbox items. Usually these issues appear in Silverlight projects.
The following steps allow you to reset the Toolbox completely and enable Visual Studio to rebuilt it from scratch. In most cases, this procedure solves all issues related to Silverlight toolbox items.
Above all, reset the Toolbox from Visual Studio. For this right-click the Toolbox and click the "Reset Toolbox" item. After the reset procedure is done, follow these steps:
1. Close Visual Studio;
2. Open the "c:\Users\<your_user_name_here>\AppData\Local\Microsoft\VisualStudio\10.0" (Windows 7) folder and remove all the .TBD files;
3. Run the "regedit" tool. For this click the "Run" item in the Start menu and type "regedit" without quotation marks;
4. Find the "HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\10.0\ToolboxControlsInstaller_AssemblyFoldersExCache" and "HKEY_CURRENT_USER\Software\Microsoft\VisualStudio\10.0\ToolboxControlsInstallerCache" keys;
5. Remove everything from these keys leaving them empty;
6. Run Visual Studio again and wait until it recreates all items in the Toolbox.
K18430: How to resolve issues with Toolbox items (Applies to versions prior to 12.1)