COMException while accessing .NET WebBrowser (SHDocVw)

What is the reason for the following exception ?

System.Runtime.InteropServices.COMException (0x800706BA): The RPC server is unavailable. (Exception from HRESULT: 0x800706BA)

It occurs when I make a COM 开发者_开发知识库call, trying to manipulate InternetExplorer in a Windows XP machine running c# code. It raises when I try to read a property like .IsBusy.

Exception message: Unable to cast COM object of type 'System.__ComObject' interface type 'SHDocVw.IWebBrowser2'. This operation failed because the QueryInterface call on the COM component for the interface with IID '{-D30C1661 CDAF-11D0-8A3E-00C04FC9E26E}' failed due to following error: The RPC server is unavailable. (Exception from HRESULT: 0x800706BA)