About GAC

http://blogs.msdn.com/b/msbuild/archive/2007/04/12/new-reference-assemblies-location.aspx

http://weblogs.asp.net/soever/archive/2008/07/20/c-program-files-reference-assemblies-for-assemblies-to-reference-in-your-code.aspx

C:\Program Files\Reference Assemblies

 

http://msdn.microsoft.com/en-us/library/yx7xezcf%28v=vs.71%29.aspx