LabVIEW cannot directly write to an XLS format file.
You can write a CSV and open it in Excel (Excel will open a CSV file with a .xls extension, as you are seeing).
On Windows you can use the Report Generation Toolkit or one of the Excel automation interfaces (.NET/ActiveX) to tell Excel to write an XLS file with the values that you specify programatically. This requires that Excel be installed on the local machine.
In LabVIEW 2013 you can use the "Write to Measurement File Express VI" to write directly to XLS*X* files without having Excel installed.
There may be a non-NI support library or command line tool you could call but I'm not aware of one.
This document explains a little more: http://www.ni.com/newsletter/51339/en/