Set up normal.dot as your letterhead template. However, be aware that there
are downsides to this: Word creates some documents automatically using
normal.dot -- such as labels and envelopes; so while it might simplify your
letter-writing, it will likely interfere with your other uses of Word (if
any).
An alternative is to create a separate letterhead template, then set up a
desktop shortcut to open Word with a new document using that template:
Winword.exe /t letterhead.dot