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var
  a : Byte;
  s : string;
  h : THandle;
begin
  for a:=1 to 4 do
  begin
    s:='COM'+InttoStr(a)+':';
    h:=CreateFile(PChar(s),
                  GENERIC_READ or GENERIC_WRITE,
                  0,
                  nil,
                  OPEN_EXISTING,
                  FILE_ATTRIBUTE_NORMAL,
                  0);
    if h <> INVALID_HANDLE_VALUE then
    begin
      ListBox1.Items.Add(s);
      CloseHandle(h);
    end;
  end;
end;