Hopefully this will save me or someone else hours of frustration in the future. As I was trying to design a new web page today, I found out I couldn’t drag any of the controls from the Toolbox to the editor in my new project in Visual Studio. Scratching my head, I scoured the web for answers, and ultimately decided to re-install it according to the instructions found on Microsoft’s AJAX site. It worked, and then I realized what the problem might have been. I had performed one of those automatic Visual Studio updates to update the Ajax Control Toolkit or had re-installed a newer version sometime. It might have been trying to reference the assembly of an older version, or it couldn’t access the .dll which was stored in another project. But now, you at least have a possible solution to your problem if you are experiencing the same thing!