I just downloaded the latest beta version of ArcGIS Viewer for Silverlight and I'm getting the attached message box when trying to install. I have IIS enabled and the prerequisite checks for features. I'm running 64-bit Windows 7.
The error message you're getting was identified as a bug in the ArcGIS Viewer for Silverlight beta 2 setup. A workaround to address this issue was posted on the Esri Beta Community forums:
In future, please post comments and questions related to the Silverlight Viewer beta 2 on the Esri Beta Community, as this is where the Dev team is monitoring user feedback.