The Problem:-
Media Center keeps shutting down with a problem event CLR20r3.
Cause:-CLR20r3 is associated with .net framework.
Resolution:-
To fix the issue uninstall .NET Framework.There is a .NET Framwork removal tool,developed by Aaron Stebner.You can download it here:-
http://blogs.msdn.com/astebner/archive/2005/04/08/406671.aspx
After uninstalling it,reinstall the same version of .NET Framwork you have just uninstalled.