Sometimes the Visual Studio 2010 toolbox cache can get a bit corrupted. Controls that you once were able to drag onto a form no longer seem alive, or you may start seeing double, triple and quadruple entries for the same controls. Unfortunately, the toolbox logic is a bit buggy in VS2010 and corrupted caches can frequently occur. Especially for WPF and Silverlight as these platforms require new and different methods for adding items to the toolbox than sturdy WinForms or ASP.NET. These newer frameworks are limited to allowing 3rd party control vendors to just set specific registry entries and then we rely on Visual Studio to add items to the toolbox from there.
The good news is you can resolve all your toolbox issues without having to uninstall or reinstall anything. The solution is to simply delete your toolbox cache.
On Windows XP:
\Documents and Settings\<user directory>\Local Settings\Application Data\Microsoft\VisualStudio\10.0\*.tbd
On Vista/Windows 7:
Removing all .tbd files will clean up your corrupted cache. When you restart Visual Studio your toolbox will be reset, duplicate items gone and the cache will start over. When the toolbox cache is reset it looks through your registry for toolbox entries to add (such as Studio for Silverlight). Any controls that you added manually will have to be added back using the standard approach (Tools - Choose Toolbox Items). In some cases with WPF you may lose other 3rd party controls which you will have to add back manually as well.
Unfortunately, there's little we can do to prevent cache corruption. Microsoft has been aware of these issues and will probably address them in VS2011+.