Maybe you used to be able to open up Excel files in ArcCatalog and ArcMap, or maybe your new install will not open them. You get this great message when you try to open Excel files:
Failed to connect to database. An underlying database error occurred. Class not registered
This happens when your IT group moves you to Microsoft Office 365 in the cloud, or you never had Microsoft Office on your computer. ArcCatalog and ArcMap depend on certain Microsoft Office system drivers to be installed on your computer which allows you to open Excel files in your GIS. Sure would be nice if the error message would tell you that!
So how to fix it? Just install the 32bit Microsoft Access Database Engine. It will install a set of components that will be used to facilitate the transfer of data between 20xx Microsoft Office System files and non-Microsoft Office applications such as ArcCatalog and ArcMap.
Remember, install the 32bit version, not 64bit, since ArcGIS Desktop is a 32bit application.
SoCalGIS.org 
Our students run into this one often. Another work around if they are using a computer where they can’t install the fix is to open the file in Excel and save as the older version that gives an XLS extension. ArcMap works with that.
That’s a good one!