Have you tried to remove the old installation, clean the GAC and if this does not help could you try to move away the design dlls from the installation folder. Don't delete them just move them to your desktop for example, close and open the studio and if this does not fix your problem move them back.
the Telerik team
Register for the Q2 2011 What's New Webinar Week. Mark your calendar for the week starting July 18th and book your seat for a walk through of all the exciting stuff we will ship with the new release!