If you are getting "The operation could not be completed. Invalid Formatetc Structure" error while using Asp.Net Ajax controls in Visual studio 2008 then this error occurs because of version conflict of System.web.extensions assembly installed in GAC.
To fix this problem try any of the following solutions.
Visual studio 2008 uses .NET framework version 3.5 and System.web.extensions version 3.5.
This error occurs because of two versions of System.web.extensions installed in Global assembly cache (GAC) typically in C:\WINDOWS\assembly
Visual studio 2008 automatically installs System.web.extensions version 3.5 in GAC and if you also uses visual studio 2005 or have installed asp.net ajax extensions for Visual studio 2005 which installs System.web.extensions version 1.0.xxxxx.x in GAC and hence version conflict and this Invalid FORMATETC structure error occurs as shown below..
The simplest solution to fix this error is to put Latest version of AjaxControlToolkit.dll for .Net framework 3.5 in BIN folder of website or application.
Uninstall or remove System.web.extensions version 1.0.xxxxx.x from GAC which was installed to use ajax controls with visual studio 2005.