I would suggest that you upgrade the Telerik WinForms VSExtensions to their latest version. It automatically deploys the design time assembly to the GAC when the Upgrade wizard run, hence I hope both the problems will get solved.
Please, try the following steps:
1) Backup your project just in case
2) Remove the reference to the Telerik assemblies from your project (note their list for reference)
3) Close all Visual Studio instances
4) Use Windows Explorer to check the following two folders for Telerik subfolders and delete them:
5) Visit the Visual Studio Online Gallery and download the Telerik WinForms VSExtensions
to a folder of your choice
6) Start a Visual Studio 2010 Command Prompt As Administrator and navigate to the folder you downloaded the VSIX file to
7) Run the following command:
vsixinstaller /a [TelerikWebUIVSIX_FileName.vsix]
9) Start Visual Studio and load your project
10) Right-click your project in Solution Explorer, use the RadControls for WinForms -> Convert to Telerik Application
menu item and follow the wizard to add the references to the selected RadControls distribution (version).
Please, perform steps 6) -> 8) for any other Telerik VSExtensions you would need to have.
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!