Problems to install silverlight
I had this problem a while back. What was suggested and worked for me was a full uninstall of Silverlight, a reboot, and a reinstall. You may have to manually remove Silverlight files.

Here's a good explanation of the issue and how to fix it: http://answers.microsoft.com/en-us/mac/forum/macoffice2011-macother/netflix-silverlight-mac-problem-installation-says/0b71af78-cb94-46e3-9629-f97c8dda1361