The first thing you should do here is to close all programs that are using the Excel file you are attempting to open. Please ensure the .xls file is not opened in other workbooks, add-ins, or apps.

If Task Scheduler is not opening an Excel file on your Windows PC, ensure the task is set to run only when user is logged on, or use these fixes.

This month's monster security patch for Office breaks Excel when it tries to open HTML files, and it seems unlikely Microsoft will fix the problem ...