protected void Page_Load(object sender, EventArgs e) {
    DataTable table = GetDataTable();
    if (!IsPostBack)
        CreateTreeViewNodesRecursive(table, this.treeView.Nodes, "0");
}

private void CreateTreeViewNodesRecursive(DataTable table, TreeViewNodeCollection nodesCollection, string parentID) {
    for (int i = 0; i < table.Rows.Count; i++) {
        if (table.Rows[i]["ParentID"].ToString() == parentID) {
            TreeViewNode node = new TreeViewNode(table.Rows[i]["Title"].ToString(), table.Rows[i]["ID"].ToString());
            nodesCollection.Add(node);
            CreateTreeViewNodesRecursive(table, node.Nodes, node.Name);
        }
    }
}