If your download does not start after 30 seconds,
Before installing this Silverlight 3 SDK, please remove any previous Silverlight 3 SDK install.
If you have installed or plan to install Microsoft Silverlight Tools for Visual Studio 2008, please skip installing the Silverlight SDK directly. Silverlight 3 SDK is installed by Microsoft Silverlight Tools. For more on that click here
To build Silverlight 3 applications, you can install Silverlight 3 for Developers here