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;