Open a new workbook.  Customize as you wish.

File>Save As Type:  scroll down to Excel Template(*.XLT) and select.  Name your
workbook "BOOK"(no quotes). Excel will add the .XLT to save as BOOK.XLT.

Store this workbook in the XLSTART folder usually located at........

C:\Documents and Settings\username\Application Data\Microsoft\Excel\XLSTART

This will be the default workbook for File>New or the Toolbar button File>New

WARNINg................Do not use File>New...Blank Workbook or you will get the
Excel default workbook.

NOTE:  Existing workbooks are not affected by these settings.

You can also open a new workbook and delete all but one sheet.  Customize as
you wish then save this as SHEET.XLT in XLSTART folder also.  It now becomes
the default Insert>Sheet.

More can be found on this in Help under "templates"(no quotes).

Gord Dibben   Excel MVP