Requires installing the Solr Search Module:  http://drupal.org/project/apachesolr .

Once you have your Solr server up and running, and have configured it to work with Drupal at /admin/settings/apachesolr, you must configure the Drupal Search Form to work with Solr as follows:

<?php
/**
* Implementation of hook_form_alter()
*/
function your_custom_module_form_alter(&$form, $form_state, $form_id) {
  if ($form_id == 'search_theme_form') {
    $form['#submit'][] = 'your_custom_module_search_submit';
  }
}

/**
* Re-direct Search Form Submit
*/
function your_custom_module_search_submit($form, &$form_state) {
 $form_id = $form['form_id']['#value'];
 $form_state['redirect']='search/apachesolr_search/'. trim($form_state['values'][$form_id]);
}
?>