Please ensure that your firewall will allow for program installation.
Please try downloading the installer on Internet Explorer and be sure to save the installer, not simply run it.

Since Silverlight is not a Mozilla product, we really can't provide the best assistance.
For further assistance, please consider contacting Microsoft.

Also, please ensure that you are downloading Silverlight from the official website.

The Silverlight installer is saying it cannot run on a Windows 8 computer?? That's strange. Do you get a specific error message?

If Silverlight stopped part of the way through installation, you might need to cleanly remove it and then try installing again. This Microsoft article has the steps for that: How to clean a corrupted Silverlight installation and then reinstall Silverlight.