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 ...

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.

If you ever decide to return to the out-of-the-box condition, just go back to the XLStart directory and remove the Book1.xls file. Excel will generate a new template and open with its factory ...