Can't install Silverlight Developer runtime

Solution 1:

I was getting the same error. The problem was that the link that Visual Studio was giving was for the 32 bit version, and I am running 64 bit. Some kind of error mentioning that when failing the install would have been nice...

Here is the link for the 64-bit version of Silverlight 5 developer tools:
http://go.microsoft.com/fwlink/?LinkID=229324

Solution 2:

Service Pack 1 needed for Silverlight 5.

I suggest to(based on my recent experience)

1. Uninstall everything "Microsoft Silverlight" that you have
2. Make sure SP1 installed
3. Install Silverlight 5 tools from here: http://www.silverlight.net/