Enabling Default Widgets in a Custom Theme

that’s not the way, wp_register_sidebar_widget is used to create widgets.
if you want your sidebar to display widgets by default use [dynamic_sidebar()][1] in your theme eg from the 2010 theme:

<div id="primary" class="widget-area" role="complementary">
            <ul class="xoxo">

<?php
    /* When we call the dynamic_sidebar() function, it'll spit out
     * the widgets for that widget area. If it instead returns false,
     * then the sidebar simply doesn't exist, so we'll hard-code in
     * some default sidebar stuff just in case.
     */
    if ( ! dynamic_sidebar( 'primary-widget-area' ) ) : ?>

            <li id="search" class="widget-container widget_search">
                <?php get_search_form(); ?>
            </li>

            <li id="archives" class="widget-container">
                <h3 class="widget-title"><?php _e( 'Archives', 'twentyten' ); ?></h3>
                <ul>
                    <?php wp_get_archives( 'type=monthly' ); ?>
                </ul>
            </li>

            <li id="meta" class="widget-container">
                <h3 class="widget-title"><?php _e( 'Meta', 'twentyten' ); ?></h3>
                <ul>
                    <?php wp_register(); ?>
                    <li><?php wp_loginout(); ?></li>
                    <?php wp_meta(); ?>
                </ul>
            </li>

        <?php endif; // end primary widget area ?>
            </ul>
        </div><!-- #primary .widget-area -->