$OSCOM_CategoryTree = new category_tree();
$OSCOM_CategoryTree->setParentGroupString('<ul>', '</ul>', true);
$category_tree = $OSCOM_CategoryTree->getTree();

outputs:

<ul>
  <li>
    <a href="/index.php?cPath=1">Hardware</a>
    <ul>
      <li><a href="/index.php?cPath=1_17">CDROM Drives</a></li>
      <li><a href="/index.php?cPath=1_4">Graphics Cards</a></li>
      <li><a href="/index.php?cPath=1_8">Keyboards</a></li>
      <li><a href="/index.php?cPath=1_16">Memory</a></li>
      <li><a href="/index.php?cPath=1_9">Mice</a></li>
      <li><a href="/index.php?cPath=1_6">Monitors</a></li>
      <li><a href="/index.php?cPath=1_5">Printers</a></li>
      <li><a href="/index.php?cPath=1_7">Speakers</a></li>
    </ul>
  </li>
  <li>
    <a href="/index.php?cPath=2">Software</a>
    <ul>
      <li><a href="/index.php?cPath=2_19">Action</a></li>
      <li><a href="/index.php?cPath=2_18">Simulation</a></li>
      <li><a href="/index.php?cPath=2_20">Strategy</a></li>
    </ul>
  </li>
  <li>
    <a href="/index.php?cPath=3">DVD Movies</a>
    <ul>
      <li><a href="/index.php?cPath=3_10">Action</a></li>
      <li><a href="/index.php?cPath=3_13">Cartoons</a></li>
      <li><a href="/index.php?cPath=3_12">Comedy</a></li>
      <li><a href="/index.php?cPath=3_15">Drama</a></li>
      <li><a href="/index.php?cPath=3_11">Science Fiction</a></li>
      <li><a href="/index.php?cPath=3_14">Thriller</a></li>
    </ul>
  </li>
  <li>
    <a href="/index.php?cPath=21">Gadgets</a>
  </li>
</ul>