I've been through this although the solution is one line of code but I had a hard time to find it so I thought it's better to share it with u guys,
in every page or user control that u have that are using Telerik Rad Controls put the Following line in the Page_Load event
protected void Page_Load(Object sender,EvenArgs e)
{
RadControlName.EnableAjaxSkinRendering=true;
//for example if u have a Rad Combo box put
//RadCombo1.EnableAjaxSkinRendering=true;
}
I hope that would help