Microsoft Ribbon for WPF - problem under Windows XP

Hello everyone!

I'm having a problem with WPF Ribbon Application under Windows XP. Applications window borders shows up weirdly and ugly. I think screenshot will describe my problem best. This is a screenshot from sample project supplied with Microsoft Ribbon for WPF installation package:

http://public.blu.livefilestore.com/y1phEQXvhrOL-zTeUUDhL4W9EBxpGG2SoaCGQXiKxju2Qm6m16kiQ15Ke6wtQ6GMOS2hTs2FgvRo3ef5oXJcyaUFw/wpf_ribbon_problem.JPG?psid=1

I'm developing with Visual Studio 2010 and C#-language. .NET framework 4.0 installed. The problem doesn't happen with normal WPF Application.

My client wants ribbon-like user interface and they are still using Windows XP. I would greatly appreciate any help as I'm running out of my own ideas.

Sincerely Otto Patolinna