Normal.dot, Normal.dotm, and Excel*.xlb are files that belong to Microsoft Office. They hold a variety of user settings pertaining to selected default fonts, default templates, active add-ins, and more. The information written into these files comes from many different sources. Over time, these files can become fragmented or corrupt, resulting in Word and Excel errors.

Microsoft does not publicly document what is stored within these files, or what format the information is stored in. When an error occurs as a result of one of these files going corrupt, it is generally not possible to determine what is wrong with the file, since we do not have access to Microsoft’s technical knowledge and documentation about the content of the files. In this case, it is easiest to simply delete the existing file. The next time Microsoft Word or Excel are opened, a new copy of the necessary files will be created.