procedure TForm1.Button1Click(Sender: TObject);
var
        UserFree, DiskTotal: Int64;
        PDiskFree: PLargeInteger;
begin
    GetDiskFreeSpaceEx( 'C:\', UserFree, DiskTotal, @PDiskFree );
    ShowMessage( format('利用可能容量は%.2fGBです。',
                    [(((UserFree/1024.0)/1024.0)/1024.0)]) );
end;