You have a global style for TD / TR / LI or UL elements that interfere with RadWindow's skins. Please check your stylesheets and if you cannot locate the selector that is causing this, send a live Url or a sample project where this issue could be reproduced. I will check it and get back to you right away.
the Telerik team
Do you want to have your say when we set our development plans?
Do you want to know when a feature you care about is added or when a bug fixed?
Telerik Public Issue Tracking
system and vote to affect the priority of the items.