This morning something weird happened to me. Every time I opened an Excel document, an error message window popped-up.

After googling for a while, I realized that it is related to Excel’s Add-Ins. You can check the Excel’s Add-Ins window (Tools -> Add-Ins), and disable any Add-Ins that previously were not there. Simply un-check the checkbox of the respective Add-Ins and click OK to finalize the changes.

In my case, I was recently installed Crystal Report and it has an Excel Add-Ins.