{$IFNDEF WIN32}
const
  WF_WINNT = $4000;
{$ENDIF}

function IsNT : bool;
{$IFDEF WIN32}
var
  osv : TOSVERSIONINFO;
{$ENDIF}
begin
  result := true;
  {$IFDEF WIN32}
  GetVersionEx(osv);
  if osv.dwPlatformId = VER_PLATFORM_WIN32_NT then
    exit;
  {$ELSE}
  if ((GetWinFlags and WF_WINNT) = WF_WINNT ) then
    exit;
  {$ENDIF}
  result := false;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  if IsNt then
    ShowMessage('Running on NT')
  else
    ShowMessage('Not Running on NT');
end;