It's not a question of which is the best version .NET, CloD needs version 4 to run.
Make sure you have
all versions of .NET installed correctly.
http://blogs.msdn.com/b/astebner/arc...3/8999004.aspx
http://blogs.msdn.com/b/astebner/arc...of-the-os.aspx
It would also help to send a crash log to the dev team.