When it starts, Excel will try to open any (and all) document files in your
XLStart folder. The default location in XP is:
C:\Program Files\Microsoft Office\Office10\XLStart
If you create your own "default" workbook or worksheets templates, this is
also where you would place your Book.xlt and Sheet.xlt files. Those
templates would then be used (almost) every time you created a new workbook
or inserted a new sheet. (One exception that I know of is that when
generating individual pages from a Pivot table's page field, any custom
sheet template is ignored. At least this used to be the case, I haven't
tested it in a few years. Because of this I wouldn't be surprised if there
are other exceptions: while under Automation from another application, etc.)