>> To the best of my knowledge (I'm only your fellow Photosynther - not anyone from Microsoft) Silverlight only works within the 32 bit version of Internet Explorer.
>> On Windows 7 64 bit, you have both the 32 bit and the 64 bit versions of Internet Explorer, but you'll need to use the 32 bit IE to run most plugins, Silverlight included.
>> Since Silverlight is only offered in 32 bit, it should automatically install to C:\Program Files (x86)\... (where all 32 bit apps are installed on any 64 bit version of Windows) rather than C:\Program Files\...
>> Your currently installed version of Silverlight ( 3.0.4536.1000 ) definitely needs to be updated to the current version of Silverlight ( 3.0.40818.0 ). If necessary, uninstall the current version, download the installer for the most recent version from http://www.microsoft.com/silverlight/get-started/install/default.aspx (installer should only be 4.7 Megabytes), and flush your browser's cache.