Magento - Display only 'root category' in top menu

 <?php

  $root_category = Mage::getModel('catalog/category')->load(3); // Put your root category ID here.
  $subcategories = $root_category->getChildren();
  foreach(explode(',',$subcategories) as $subcategory) {
        $category = Mage::getModel('catalog/category')->load($subcategory);
        echo '<a href="'.$category->getURL() .'" />'.$category->getName().'</a><br/>';
  }

  ?>