You need to create a new, default workbook template and/or a new, default
worksheet template.
Open a new blank workbook and set up the footer for all of the worksheets
(and whatever else you find yourself changing for every workbook – in my own
I have also switched the layout to landscape, for example). Click Save as
and change the file type to Excel Template (*.xltx). Change the file name to
book.xltx and save it in the XLSTART folder under your office installation
folder. (Usually C:\Program Files\Microsoft Office\Office12\XLSTART). This
will take care of any new workbooks you open, if you also want it to apply to
new worksheets added to a workbook, however, delete all but the first sheet
in a new blank workbook, set that sheet up as desired and save it as
sheet.xltx in the same folder.