Автор: http://www.swissdelphicenter.ch

uses
   ComServ, SysUtils;

 function GetDirectory: string;
 // This function returns the directory where the DLL (or ocx) is installed. 
// Also works in webservices 
// With DLL's you cannot use "Paramstr(0)" because this will give a false value 
begin
   Result := IncludeTrailingBackslash(ExtractFileDir(ComServer.ServerFileName));
 end;