A family of Microsoft word processing software products for creating web, email, and print documents.
Create the letterhead and then save it as a “Word template.” At the “Save As” window save in the "Templates" folder. At "File name:" box" type "letterhead," as name of template, and then in the “Save as type:” box, click on the drop down arrow and choose “Word template.”
For a more detailed explanation go to:
http://office.microsoft.com/en-gb/word-help/save-a-word-document-as-a-template-HP005189265.aspx