How do I determine the available disk space on a drive?

function GetAvailableSpace(DriveLetter: Char): String;
var
  DiskCapacity, FreeSpace: Extended;
begin
  DriveLetter := UpCase(DriveLetter);
  DiskCapacity := DiskSize(Ord(DriveLetter) - 64) / 1048576;
  FreeSpace := DiskFree(Ord(DriveLetter) - 64) / 1048576;
  if DiskCapacity <= 0 then
    Result := 'Disk information not available'
  else if FreeSpace > 1000 then
  begin
    DiskCapacity := DiskCapacity / 1024;
    FreeSpace := FreeSpace / 1024;
    Result := FormatFloat('#,##0.00', FreeSpace) + 'GB of ' + FormatFloat('#,##0.00', DiskCapacity) + 'GB';
  end
  else
    Result := FormatFloat('#,##0', FreeSpace) + 'MB of ' + FormatFloat('#,##0', DiskCapacity) + 'MB';
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  driveLetter: Char;
begin
  for driveLetter := 'B' to 'Z' do
  begin
    if GetDriveType(PChar(driveLetter + ':\')) = DRIVE_FIXED then
      ValueListEditor1.InsertRow('Available Space (' + driveLetter + ':)',
       GetAvailableSpace(driveLetter), True);
  end;
end;