function DriveExists(Drive: Byte): boolean;

begin

  Result := Boolean(GetLogicalDrives and (1 shl Drive));

end;

procedure TForm1.Button1Click(Sender: TObject);

var Drive: byte;

begin

  for Drive := 0 to 25 do

    if DriveExists(Drive) then

      ListBox1.Items.Add(Chr(Drive + $41));

end;