It looks that you have some global style on your page which interferes with the RadWindow control. Can you please try isolating the problem in a small sample project and send it to us? We will check it right away and do our best to help.
To be able to answer that I need to have a project which reproduces your setup and the behavior that you experience. That is why I would like to repeat my request to send me a sample project - once I have a better view over your case and the style that you are using, I will get back to you with solution right away.