Take my word for it, you don't really want anything other than Normal.dot to
be "the" default template. It's used for various other things, such as
sheets of labels, that would be messed up if they had a letterhead in them.
Instead, right-click your template file in Windows Explorer and select Send
To > Desktop (create shortcut). When you want a letterhead "blank",
double-click that icon on the desktop. When you want anything else, use the
original Word icon.