What is the best graphic format for importing images into Microsoft Word?

hello, I have used adobe illustrator to create a letterhead with logo. Now I like to save this leterhead as template in ms word. What is the best graphic format that I can use to bring my leterhead in to word and save it in a template so that my client type his letters right on his letterhead and print it to his laser printer? I have uses eps in the past and it used to work alright, but now I have now it is kind of alright on the mac but when I open the same file in a window pc using office 2003 and window xp. The eps image shows in black and when I print it also prints black and it looks really bad in print. I mac it look bad on screen but prints fine. My client is using window xp so I have to make it look good and print good from windows.

Thanks,
omar