procedure TForm1.Button1Click(Sender: TObject);
var
  ld: DWORD;
  i: integer;
begin
  ld := GetLogicalDrives;
  for i := 0 to 25 do 
  begin
    if (ld and (1 shl i)) <> 0 then
      Memo1.Lines.Add(Char(Ord('A') + i) + ':\');
  end;
end;