As the option seems to be widely used we will make this choice available again in our next official release.
Meanwhile you could use the following workaround:
- Open a Command prompt
- Navigate to the location of the Telerik RadControls installer
- Use the following command line: msiexec /i Telerik.Web.UI_2010_3_1109_Dev.msi SKIPINSTALLAJAXEXTENSIONS="true" (this way the installation of ASP.NET AJAX 1.0 will be skipped)
Just for your information in the following forum post a similar problem is discussed: http://www.telerik.com/community/forums/aspnet-ajax/general-discussions/q3-installation-choices-for-net-version.aspx
the Telerik team