uses
  windows, ddeman, ...


function Get_URL(Servicio: string): string;
var
  Cliente_DDE: TDDEClientConv;
  temp: PChar;      //<<-------------------------This is new
begin
  Result := '';
  Cliente_DDE:= TDDEClientConv.Create( nil );
  with Cliente_DDE do
  begin
    SetLink( Servicio,'WWW_GetWindowInfo');
    temp := RequestData('0xFFFFFFFF');
    Result := StrPas(temp);
    StrDispose(temp);  // <<-- Предотвращаем утечку памяти
    CloseLink;
  end;
  Cliente_DDE.Free;
end;

procedure TForm1.Button1Click(Sender);
begin
   showmessage(Get_URL('Netscape'));
// или
   showmessage(Get_URL('IExplore'));
end;