If you created a default workbook template, you should be able to find it in
your XLStart folder--it'll be named book.xlt.
(There may be another template in there named sheet.xlt that contains a template
when you add a new worksheet to an existing workbook.)
You could either delete that book.xlt template file or even open it and modify
it.
And be careful. Depending on how you upgraded excel, you may have multiple
XLStart folders and multiple book.xlt files. I'd use windows start
button|search to look for all of them.