Got to the bottom of my WatiN issues when trying to run a simple test on our Windows Server 2008 development machine with IE8. I kept getting the following error:
System.Runtime.InteropServices.COMException : The interface is unknown. (Exception from HRESULT: 0x800706B5)
Thankfully SO can to the rescue again: http://stackoverflow.com/questions/1014815/how-do-you-get-watin-to-work-on-windows-server-2008-with-ie8
Run VS as Administrator!