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.

If Excel is not set as the default application to open the Excel file types (.xls, .xlm, .cvs, etc.), the file may open in another application. Press Win+I key combinations to open Windows Settings.

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.

Step 1: In Excel, open the document you want to secure with a password. Step 2: Click File, followed by Info. Mark Coppock / Digital Trends ...