Displaying partial menu from sitemap

Hello
 I want to display only the parent nodes from the sitemap in the main horizontal navigation bar. Depending upon the node selected, i want to display the child nodes coresponding to the selected parent node as a vertical menu below the main nav bar.
 most of the books only explain how to access full sitemap to display in menu bar.
Right now am using only the old fashioned way of creating menu's , just simple html coded menu in asp.net.
 following is the example of sitemap to be used.
 <?xml version="1.0" encoding="utf-8" ?><siteMap xmlns="http://schemas.microsoft.com/AspNet/SiteMap-File-1.0" >    <siteMapNode url="~/" title="Home"  description="Home">       <siteMapNode url=" ...