I am assuming you know that the Write to Spreadsheet File VI will not create an Excel workbook just because you use an extension of .xls. You get a text file. The extension will simply allow your operating system to automatically open Excel when you double-click on the file.
That said, %.s is not the format string you want to use for floating point values. You should use %f, or something else that converts a number to a string. The default format string is %.3f, which uses 3 decimal places.
If you have a string that you want to include then one simple way to do this is to convert the numerical array to a string (using the number to string functions), prepend your string to this string array (i.e., inserting an element to the beginning of the array), and then writing out this new string array. The Write to Spreadhsheet File is polymorphic in 8.2 so you'll get the string version when you connect a string array to it.
