Install the '.NET Framework Version 2.0 Software Development Kit' from <a href="http://msdn.microsoft.com/en-us/netframework/aa731542.aspx" target="_blank">http://msdn.microsoft.com/en-us/netframework/aa731542.aspx</a> . Once you launch the setup, select (at minimum) the ‘Tools and Debugger’ option. This will install the Microsoft.mshtml.dll interop to '\Program Files\<a href="http://Microsoft.NET" target="_blank">Microsoft.NET</a>\Primary Interop Assemblies' and then add it to the GAC. The Query task design time in Web Developer Express will then be able to find the assembly.