There are tried and true methods to set up letterhead in Word.
See http://addbalance.com/word/headersfooters.htm and
http://wordfaqs.ssbarnhill.com/LetterheadRibbon.htm

Here is a tutorial with sample documents demonstrating the concepts.
http://addbalance.com/word/download....StylesTutorial

The key is to put your letterhead elements into the First Page Header (and potentially into continuation Headers). This is true even if they are in places on the page outside of the traditional header area. This can be used to mimic different margins and insert logos.

The headers and footers are normally not the area of focus when writing letters and can be protected.

I hope this information helps.