[DllImport ("advapi32.dll")] public static extern bool GetUserName     (StringBuilder LPBuffer, ref Int32 LPNSize);
[DllImport ("kernel32.dll")] public static extern bool GetComputerName (StringBuilder LPBuffer, ref Int32 LPNSize);

protected void MostrarConta ()

{
  Int32         NC, NU;
  StringBuilder Computador, Usuario;

  Computador= new StringBuilder (256);
  Usuario   = new StringBuilder (256);
  NC        = Computador.Capacity;
  NU        = Usuario.Capacity;

  GetUserName (Usuario, ref NU);
  GetComputerName (Computador, ref NC);

  Conta.Text = "Conta     : " + Usuario + '\xA';
  Conta.Text+= "Computador: " + Computador;
}