public
  { Public declarations }
  Windir: string;
  WindirP: PChar;
  Res: Cardinal;

procedure TForm1.Button1Click(Sender: TObject);
begin
  WinDirP := StrAlloc(MAX_PATH);
  Res := GetWindowsDirectory(WinDirP, MAX_PATH);
  if Res > 0 then
    WinDir := StrPas(WinDirP);
  Label1.Caption := WinDir;
end;