I find the path of Microsoft.Office>interop.Excel under the Solution/References is
C:\WINDOWS\assembly\GAC\Microsoft.Office.Interop.Excel\11.0.0.0__71e9bce111e9429c\Microsoft.Office.Interop.Excel.dll
However, such a file doesn't exist on my C:\WINDOWS\assembly\GAC at all. Further, there is not GAC folder under C:\WINDOWS\assembly.
Any idea?
Use the command prompt to navigate there. Windows Explorer has an extension that runs for the assembly folder, hiding some of the details.
Here is a post on how to turn it off. http://geekswithblogs.net/pavelka/archive/2006/05/05/WindowsExplorerAndTheGlobalAssemblyCache.aspx or http://weblogs.asp.net/jkey/archive/2003/02/25/3006.aspx