One way would be to scan or otherwise produce your letterhead content so you
can put it into Word's first-page header or footer (see File - Page Setup -
Layout - Different First Page). Then, select just the letterhead material
(which might be a graphic, graphic + text, or just text), and choose
Format - Font - and tick the Hidden option.
Next, in Tools - Options - View, tick the Hidden text option so that the
letterhead material will display onscreen (except in Print Preview).
Finally, in Tools - Options - Print, make sure that Hidden text is NOT
checked under Include with document.
Now, your letterhead will display in Print Layout, but will not print (and
hence, can't display in Print Preview).