Your XLStart folder.
You can use windows|Find (or Search) to look for it.
Or you can do this:
Start Excel
hit Alt-F11 (to get to the VBE where macros live)
hit ctrl-G (to see the immediate window)
type this and hit enter.
?application.StartupPath
I get this with win98 and xl2002:
C:\WINDOWS\Application Data\Microsoft\Excel\XLSTART
You'll want to save this template file as book.xlt. If you want the similar
effect for new sheets to existing workbooks, create another template, save it in
the same location, but name it sheet.xlt.