Problemas com Função DiskFree()

function GetDiskFreeSpaceEx(lpDirectoryName: PAnsiChar;
  var lpFreeBytesAvailableToCaller : Integer;
  var lpTotalNumberOfBytes: Integer;
  var lpTotalNumberOfFreeBytes: Integer) : bool;
  stdcall;
  external kernel32
  name ´GetDiskFreeSpaceExA´;


procedure GetDiskSizeAvail(TheDrive : PChar;
                           var TotalBytes : double;
                           var TotalFree : double);
var
  AvailToCall : integer;
  TheSize : integer;
  FreeAvail : integer;
begin
  GetDiskFreeSpaceEx(TheDrive,
                     AvailToCall,
                     TheSize,
                     FreeAvail);
{$IFOPT Q+}
 {$DEFINE TURNOVERFLOWON}
 {$Q-}
{$ENDIF}
  if TheSize >= 0 then
    TotalBytes := TheSize else
  if TheSize = -1 then begin
    TotalBytes := $7FFFFFFF;
    TotalBytes := TotalBytes * 2;
    TotalBytes := TotalBytes + 1;
  end else
  begin
    TotalBytes := $7FFFFFFF;
    TotalBytes := TotalBytes + abs($7FFFFFFF - TheSize);
  end;

  if AvailToCall >= 0 then
    TotalFree := AvailToCall else
  if AvailToCall = -1 then begin
    TotalFree := $7FFFFFFF;
    TotalFree := TotalFree * 2;
    TotalFree := TotalFree + 1;
  end else
  begin
    TotalFree := $7FFFFFFF;
    TotalFree := TotalFree + abs($7FFFFFFF - AvailToCall);
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  TotalBytes : double;
  TotalFree : double;
begin
  GetDiskSizeAvail(´C:\´,
                   TotalBytes,
                   TotalFree);
  ShowMessage(FloatToStr(TotalBytes));
  ShowMessage(FloatToStr(TotalFree));
end;