From: Chris on 12/09/2002
This occurs in the Tools --> Customize menu option when you click the File (or another) menu and then click the "Customize" button and choose delete. It can also happen when you happen to click-drag on the menu button itself. The fix is:
1. Right mouse click on the Start menu button
2. Choose "Explore"
3. Locate the "Application Data" folder and open it
4. Locate the "Excel" folder and open it
5. Locate the file "Excel.xlb" and rename it to "excel.old"
Removing/renaming the excel.xlb file solves the problem since this is the default workbook where the menu customization is stored. Excel will generate a new file upon startup.