I'm running the absolute latest versions of Silverlight and Visual Studio 2010 Professional and built a solution that contained a WPF project. From there, I added a SketchFlow project (based on Blend 4 RC) and I can run each project within the solution just fine by setting whenever I switch between them as startup projects. From there, I added a Silverlight 4 Business Application (taking all the defaults), and when simply trying to set that as the startup project and running it, VS2010 is giving me the following error after trying to open a web page: Line: 56 (in file TestPage.aspx) Error: Unhandled Error in Silverlight Application Code: 8001 Category: InitializeError Message: Upgrade required If I choose not to debug, I get the Silverlight page saying "This page requires a more recent version of Silverlight"! Clicking "Install Now" on the popup window brings me to Mirosoft's Silverlight page where I see: "The version of Silverlight originally requested is not available. You can get a supported version from this page. This Web browser or operating system may not be compatible with Silverlight. Please review the system requirements and, if you wish to proceed, choose the link for your operating system." If I choose to upgrade anyway, I'm told that I'm running the latest version of Silverlight available. What the heck? I'm running the final versions of VS2010 Pro, Silverlight 4, and the latest version of Expression Blend 4 (RC). Why can't VS2010 run this default Silverlight Business App? Any ideas? Please?? Thanks, Lu10ntDn PS. This is on Windows 7 with UAC turned off, and ALL latest Windows Updates installed.