That error message basically indicates the installation package is corrupted. I would recommend attempting another download and trying again.
If you keep getting the error, you could use a download manager such as FlashGet or GetRight in order to ensure your download is complete.
Let me know if that helps.
the Telerik team
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 their blog feed