System.Runtime.InteropServices.COMException (0x80040154): Retrieving the COM class factory for component with CLSID {79A2867E-EA02-4683-8199-ED426387BA05} failed due to the following error: 80040154 Class not registered (Exception from HRESULT: 0x80040154 (REGDB_E_CLASSNOTREG)).
今天使用COM interop的时候，碰到了下面这个错误，研究了很久。在浏览下面这个网页的时候，找到了解决方案。
http://stackoverflow.com/questions/4663994/system-runtime-interopservices-comexception-0x80040154
在IIS7的Application Pools中，选中对应的应用程序池 -> Advanced Settings... -> Set the value of "Enable 32-Bit Applications" to True.