I found this on another board:
"You can set up a template with the margins you like. If you call it
Book.xlt and put it in you XLSTART folder, then XL will use it whenever you
create a new workbook. Similarly, Sheet.xlt will be used when you insert a
new sheet into an existing workbook. Just set up a blank workbook the way
you want it and choose File - Save As. In the type box, choose template."