You could also export the letterhead as a pdf. Then go to Word, Insert Picture from File, and select the pdf. This will place the pdf in your word doc as an image. Then go to View-Formatting Palette and select the Crop tool under the Image tab. Crop the image to just the area you want to see (ie,just the logo and address). You can position the image by carefully clicking on it and dragging. Next click on the text box tool (on the Drawing toolbar, the icon is an A with a cursor next to it). Now draw a text box and position it within your margins. This way, your text will not have to "float" over a whole page pdf. Next, save the whole document as .dot file to your Microsoft Office templates folder.
Now whenever you need letterhead, just open a new document based on your template and you are ready to go.