The Telerik ASP.NET AJAX RadControls have at their disposal various skins, through which the appearance of each control can be changed. Therefore you can set the Skin property of the slider to Vista, just as it is in the demo. For more information on the skins and how to use them, please, refer to the following help article - http://www.telerik.com/help/aspnet-ajax/introduction-controlling-appearance-overview.html
and the others under the same category.
Hope this helps.
the Telerik team