Letterhead template formatting problem with Word 2003

In a simple, basic, step by step explanation, can someone please explain how I can make a letterhead template permanently not mess up when I copy and paste new text?

Our office has a Microsoft Word letterhead template (image 1). The letterhead is wider than the standard 1 inch margins.



Unlike Word Perfect, which works fine, even when I copy and paste a small amount of text to include with this letterhead, the letterhead is fine, but the margins are incorrect (image 2).



If I include a large amount of multipage text, the letterhead gets all messed up (image 3).



In a simple, basic, step by step explanation, can someone please explain how I can make a letterhead template permanently not mess up when I copy and paste new text?

Thank you in advance.