It occurs to me that I might have given an inappropriate answer to your question. When you asked how to write an .xls (Excel) file, and mentioned that you had the option of writing an .xlsx file, but not .xls, I assumed that you were using the LabVIEW Report Generation Toolkit, which was the basis of my previous response.
When asking for help, it is helpful to us (and therefore helpful to you) to provide a little more information, including the Version of LabVIEW that you are using and a "snippet" of the relevant LabVIEW code that is giving your problems. [Note -- if you use LabVIEW's "Snippet" feature, shown on the Edit Menu, it not only shows code but also provides the LabVIEW version].
In particular, if you happen to be using LabVIEW 2014 (but not LabVIEW 2012 or earlier), you might have been trying to save your data by using Write to Measurement File, for which NI now provides the possibility to write the data in the .xlsx format. NI added this feature in LabVIEW 2013, as it happens. By this time, the .xlsx format (simpler, less proprietary) had become more-or-less "standard", so it was chosen as the "feature to add".
If you must write your data in the .xls format, do not use Write to Measurement File. On the other hand, if you have to use Write to Measurement File, you should be able to read your data even with Excel 2007, as Microsoft has a package that provides the ability to read the .xlsx format for this version of Excel.
I hope that this answers your question.
Bob Schor
