A few users of Excel are getting an annoying password prompt dialog request whenever they close Excel. Pressing cancel on the dialog just redisplays it. If this is happening to you, the immediate solution to closing the dialog is to press the ESC key many times, as fast as you can. Some users get the error message "Project is unviewable" instead of a prompt for a password. This is due to password protection on add-ins or differences in Excel versions.
The primary (but not only) software causing this problem is a COM add-in that has set an incorrect value in the registry.
Typically there are only 5 or so value entries to change. You may need to reboot your PC after doing all the changes to insure the edits are recognized. Normally they are immediately recognized, but we had one report that a user edited all LoadBehavior entries, and the problem was fixed only after doing the edits and rebooting.
The above has a very high success rate >>90%. So odds are it will solve your problem.