After the MS16-088 patch is applied, though, Excel refuses to open XLS files, showing a blank screen instead of a workbook. Poster conker123 on the TechNet Office forum described the problem ...

If your Excel files are opening in Notepad on Windows PC, here's how you can reset the settings to make .xls, .xlsx, etc files open in Excel.

Please ensure the .xls file is not opened in other workbooks, add-ins, or apps. After coming to that conclusion, simply attempt to open the file again in Excel to see if the problem no longer exists.

I tried to change the association setting for opening .XLS files to Excel in Windows 10, but it only gives me the Adobe option in the Control Panel.