This is a good tip for all who are who are running Windows 64-bit:
To be able to host a Silverlight 3.0 in a frame in a Windows Presentation Foundation (WPF) Application on 64-bit Windows, you may hit some obstacles your WPF application will load the 64 bit version of IE – which cannot load Silverlight currently.
Now, IE is really an x86 application, even when running in Windows x64. So, the actual real issue is mshtml. mshtml comes in both 32- and 64-bit flavors. Since by default the WPF application is compiled as processor agnostic (bingo!), it floats to x64 and gets the 64-bit version of mshtml.
Therefore, try this to resolve: Compile the WPF application as 32-bit. The project’s “Configuration Manager” in Visual Studio should be able to help with this.