The RadControls for WinForms installer does not prompt for a license key. In the Trial version of the WinForms suite these messages pops up randomly and there is no way to remove them, but to upgrade to the Developer (purchased) version.
Could you confirm that you have RadControls for WinForms installer, version 188.8.131.52851, downloaded from www.telerik.com
? May be you are using another of our products which uses a license key...
I am waiting for your response.
the Telerik team