Thank you for contacting us!
The reason for the failed installation is that it has failed to add some Telerik assemblies to the GAC of your machine. The installation package registers Telerik assemblies both in the .net 2.0 and .net 4.0 GAC. Can you make sure that your use has rights to add assemblies to the GAC?
I hope this helps.
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to the blog feed