Автор: http://www.swissdelphicenter.ch

function EnumProc(wnd: HWND; var count: DWORD): Bool; stdcall;
 begin
   Inc(count);
   result := True;
   EnumChildWindows(wnd, @EnumProc, integer(@count));
 end;

 procedure TForm1.Button1Click(Sender: TObject);
 var
   count: DWORD;
 begin
   count := 0;
   EnumThreadWindows(GetCurrentThreadID, @EnumProc, Integer(@count));
   Caption := Format('%d window handles in use', [count]);
 end;