Who owns the copyright on graphic design layouts prepared in MS Word?

I designed a professional layout for a retail catalog using MS Word and
custom graphics, saved as Adobe PDF for easy download. The design is used by
several clients, who save money because it's simple to change the colour
scheme, logo and product range. Each client who uses the template receives
notification that the design is copyright to me - they avoid high graphic
design fees based on this agreement.

But... one of those clients duplicated my design and refuses to pay or
remove it from her web site. She's earned thousands of dollars from stealing
my design, so I'm pretty angry about it. She says Bill Gates owns the
copyright of everything designed in Word, so I want to check before I spend
money on legal advice.

I doubt very much if Microsoft owns copyright on everything written/designed
in Word. What do you think?