Looking into that error a bit, I found the following post on silverlight.net:
Looks like a number of people have had this problem the solution here states it is in relation to all the SL installation processes leaving behind old files and references that screw up the latest version. Might be a good place to start. :)
Otherwise, what is your code and references that are leading to this? I'll test it on my machine to see if I can reproduce the error with the latest install of everything.