Bit more technical reply for tourpe
Quest web parts have dependencies on Microsoft Ajax library because 3rd party library Telerik does. It requires Microsoft Ajax library version 1.0.61025.0 and the following entry to be added to the web.config <httpHandlers> section.
<add verb="GET,HEAD" path="ScriptResource.axd" type="System.Web.Handlers.ScriptResourceHandler, System.Web.Extensions, Version=1.0.61025.0, Culture=neutral, PublicKeyToken=31bf3856ad364e35" validate="false" />
This requires customers to install Microsoft Ajax library (select Microsoft Ajax -> Download Essential Components -> ASP.NET AJAX Extensions 1.0). Note that AJAX functionality is integrated in ASP.NET 3.5 SP1 and does not require any additional downloads but this version won't work with Telerik controls.
Start from version 5.0 of Quest web parts, we will automatically incluce Microsoft AJAX library and will not require customers to download and install it separately.
When ASP.NET AJAX Library 1.0 is not installed, user may not be able to add Quest Web Parts and see error messages such as the following.
When Quest Web Parts are installed on SharePoint server that has ASP.NET AJAX Library 3.5 installed or part of the operating system, the following error message may appear:
Unable to cast object of type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection' to type 'System.Web.Configuration.ScriptingScriptResourceHandlerSection'.
You may find discussion threads on this such as ASP.NET Forums.
For a particular insance of this error, where customer has .NET 3.5 and Sitrion server along with SharePoint server, the web.config has all ASP.NET 3.5 settings with Quest web parts added Ajax 1.0 settings, this error is fixed by adding the following entry to the end of <runtime> element or swap the "oldVersion" and "newVersion".
<runtime>
<assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
<dependentAssembly>
<assemblyIdentity name="System.Web.Extensions" publicKeyToken="31bf3856ad364e35"/>
<bindingRedirect oldVersion="3.5.0.0" newVersion="1.0.61025.0"/>
</dependentAssembly>
</assemblyBinding>
</runtime>