//uses Registry
procedure TForm1.GetComPorts(const Liste:TStrings);
var
  reg: TRegistry;
  st: Tstrings;
  i: Integer;
begin
  reg := TRegistry.Create;
  try
    reg.RootKey := HKEY_LOCAL_MACHINE;
    reg.OpenKey('HARDWARE\DEVICEMAP\SERIALCOMM', False);
    st := TstringList.Create;
    try
      Liste.Clear;
      reg.GetValueNames(st);
      for i := 0 to st.Count - 1 do
        Liste.Add(reg.Readstring(st.strings[i]));
    finally
      st.Free;
    end;
    reg.CloseKey;
  finally
    reg.Free;
  end;
end;

procedure TForm1.FormCreate(Sender: TObject);
begin

   GetComPorts(ComboBox1.Items);
   ComboBox1.ItemIndex:=0;

end;