You can create a new workbook with all the settings you like...including
changing the normal style for numbers.
in xl2003, format|style...
Select Normal
Click modify and change what you want on the number tab.
Name this new workbook book.xlt and store it in your XLStart folder, then excel
will use this template file whenever you click on the new icon on the standard
toolbar.
But I bet after you do this, you won't want to use it. Dates and times are
numbers to excel. By changing the default number format, you'll be changing the
number format each time you enter a date or time.
(You can also create a file named sheet.xlt that will be used as a template
whenever you insert a new worksheet into an existing workbook.)