procedure StartPluggin;
begin
  dmNSTL := TdmNSTL.Create(nil);
end;

procedure StopPluggin;
begin
  dmNSTL.Free;
end;

procedure TDllThread.Execute;
Const DLL = 'C:\Work\Service\DLL\NSTL\bNSTL.dll';
begin
  HandleDLL := LoadLibrary(DLL);
  if HandleDLL <> 0 then
  begin
    @StartPluggin := GetProcAddress(HandleDLL, 'StartPluggin');
    @StopPluggin := GetProcAddress(HandleDLL, 'StopPluggin');
    if( (@StartPluggin = nil)or(@StopPluggin = nil) )then begin
      MessageDlg('Плагин '+DLL+ ' не соответствует требуемому.',mtInformation,[mbOk],0);
      Exit;
    end;
    StartPluggin;
  end
   else
     MessageDlg('Не найден указанный плагин - '+DLL,mtError,[mbOk],0);
MessageDlg('1',mtInformation,[mbOk],0); -- первый месадж
  repeat
   sleep(100);
  until terminated;
MessageDlg('2',mtInformation,[mbOk],0); -- второй месадж
end;

const
  MY_MESSAGE_BASE = CM_BASE + 1000;
  MY_MESSAGE1 = MY_MESSAGE_BASE + 1; // sample message 1
  MY_MESSAGE2 = MY_MESSAGE_BASE + 2; // sample message 2

type
  TPluginThread = class (TThread)
  private
    FWndHandle: HWND;
  protected
    procedure WndProc(var Message: TMessage); virtual;
    procedure Execute; override;
    procedure Idle;
    function ProcessMessage(var Msg: TMsg): Boolean;
    procedure ProcessMessages;
  private
    procedure MyMessage1(var Message: TMessage); message MY_MESSAGE1;
    procedure MyMessage2(var Message: TMessage); message MY_MESSAGE2;
  public
    property WndHandle: HWND read FWndHandle;
  end;


implementation

procedure TPluginThread.Execute;
begin
  try
    FWndHandle := AllocateHWND (WndProc);
    try
      ProcessMessages;
    finally
      DeallocateHWnd (FWndHandle);
    end;
  except
   // kill any exceptions
  end;
end;

procedure TPluginThread.ProcessMessages;
var
  Msg: TMsg;
begin
  while not Terminated and ProcessMessage (Msg) do {loop};
end;

function TPluginThread.ProcessMessage(var Msg: TMsg): Boolean;
begin
  Result := False;

  if PeekMessage(Msg, 0, 0, 0, PM_NOREMOVE) then
  begin
    if GetMessage (Msg, 0, 0, 0) then begin
      Result := True;
      if Msg.Message <> WM_QUIT then begin
        TranslateMessage(Msg);
        DispatchMessage(Msg);
      end else
        Terminate;
    end;
  end else
    WaitMessage;

  Idle;

end;

procedure TPluginThread.Idle;
begin
  { do something if you want}
end;


procedure TPluginThread.WndProc(var Message: TMessage);
begin
  try
    Dispatch(Message);
  except
    // kill exceptions;
  end;
end;

procedure TPluginThread.MyMessage1(var Message: TMessage);
begin
  // do something
end;

procedure TPluginThread.MyMessage2(var Message: TMessage);
begin
  // do something
end;

 SendMessage (MyThread.WndHandle, MY_MESSAGE1, 0, 0)

procedure TPluginThread.MyMessage1(var Message: TMessage);
begin
  StartPlugin
end;

procedure TPluginThread.MyMessage2(var Message: TMessage);
begin
  StopPlugin;
  Terminate;
end;


{На старте сервиса}
.......

  FMyThread := TPluginThread.Create (True);
  FMyThread.Resume;

  PostMessage (MyThread.WndHandle, MY_MESSAGE1, 0, 0);

....

{На остановке сервиса}

  if FMyThread <> nil then 
    SendMessage (MyThread.WndHandle, MY_MESSAGE2, 0, 0); { дождаться завершения }

  DllThread := TDllThread.Create(false);
  DllThread.WndHandle = 0;
  while DllThread.WndHandle = 0 do
    Sleep (0);
  PostMessage (DllThread.WndHandle, MY_MESSAGE1, 0, 0);

procedure TDllThread.MyMessage2(var Message: TMessage);
begin
  if HandleDLL <> 0 then StopPluggin;
  Terminate;
  FreeLibrary(HandleDLL);
end;

procedure TService1.ServiceStop(Sender: TService; var Stopped: Boolean);
begin
  if DllThread <> nil then
    SendMessage (DllThread.WndHandle, MY_MESSAGE2, 0, 0);
  Stopped := True;
end;

procedure TDllThread.DoTerminate;
begin
  if HandleDLL <> 0 then StopPluggin;
  FreeLibrary(HandleDLL);
end;

D>Думаю что все таки один сервис лучше Не будеш же ты из-за каждого плагина регистрить новый сервис

PluginHandle : Integer;
...
PluginHandle := {MyDll}CreatePlugin;
...
{MyDll}StartPlugin (PluginHandle)
..
{MyDll}StopPlugin (PluginHandle)

S>procedure TDllThread.MyMessage2(var Message: TMessage);
S>begin
S>  if HandleDLL <> 0 then StopPluggin;
S>  Terminate;
S>  FreeLibrary(HandleDLL);
S>end;

S>procedure TService1.ServiceStop(Sender: TService; var Stopped: Boolean);
S>begin
S>  if DllThread <> nil then
S>    SendMessage (DllThread.WndHandle, MY_MESSAGE2, 0, 0);
S>  Stopped := True;
S>end;
S>

S>procedure TDllThread.DoTerminate;
S>begin
S>  if HandleDLL <> 0 then StopPluggin;
S>  FreeLibrary(HandleDLL);
S>end;
S>

D>>Думаю что все таки один сервис лучше Не будеш же ты из-за каждого плагина регистрить новый сервис