I have now gone over to Visual Studio 2010 Beta 2
I have used it for a few days, but now I can no longer add references from the GAC.
Sorry since this is not directly a development question, but I was not sure where to ask.
Does someone know how to fix this?
Check the Global Assembly Cache. You may have done something very very bad.
EDIT
I noticed you are using Silverlight:
C:\Program Files\Reference Assemblies\Microsoft\Framework\Silverlight\v4.0
I cannot find the Microsoft.CSharp.dll I can only find Microsoft.VisualBasic.dll to use. Where is the C# dll located for Silverlight 4? I wasn’t able to find it on my computer.
taken from: http://alpascual.com/blog/a-few-issues-with-silverlight-4-after-installing-visual-studio-2010/