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...
Here is a tutorial with sample documents demonstrating the concepts.
http://addbalance.com/word/download.htm#Letterh...
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.
Links to non-Microsoft Websites
The links are to a pages on my (non-Microsoft) website and that of a Word MVP, Suzanne Barnhill. These are safe. I am providing them because I think they contain information that will help you. However, as an Independent Advisor I am required to add the following when posting here with a link to any non-Microsoft site:
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product advertised on the site before you decide to download and install it.
I hope this information helps. Please let me know if you have any more questions or require further help. If above didn't work, I will supply more troubleshooting steps to solve this problem.
Regards