Microsoft hasn’t developed a 64 bit runtime of the Silverlight Framework. This means that you have to install the 32 Bit runtime in order to see your Silverlight Applications in Microsoft Dynamics CRM 2011. So when I upgraded to Internet Explorer 9 I came across a slight issues. As my Operating System is 64 bit I couldn’t use the 32 bit installer for Internet Explorer 9. If I let the auto installer do its thing I end up with the 64 bit version!