http://maestric.com/doc/php/codeigniter_404
google一下，找一个国外朋友提供的一个解决方法。

其他几个解决方法。

http://codeigniter.com/forums/viewthread/76340/

http://codeigniter.com/forums/viewthread/90613/

http://codeigniter.com/forums/viewthread/54807/

把MY_Router类放到你的“./application/libraries/MY_Router.php”

<?php if ( ! defined('BASEPATH')) exit('No direct script access allowed');

class MY_Router extends CI_Router {

	var $error_controller = 'error';
	var $error_method_404 = 'error_404';

    function My_Router()
    {
        parent::CI_Router();
    }

	// this is just the same method as in Router.php, with show_404() replaced by $this->error_404();
	function _validate_request($segments)
	{
		// Does the requested controller exist in the root folder?
		if (file_exists(APPPATH.'controllers/'.$segments[0].EXT))
		{
			return $segments;
		}

		// Is the controller in a sub-folder?
		if (is_dir(APPPATH.'controllers/'.$segments[0]))
		{
			// Set the directory and remove it from the segment array
			$this->set_directory($segments[0]);
			$segments = array_slice($segments, 1);

			if (count($segments) > 0)
			{
				// Does the requested controller exist in the sub-folder?
				if ( ! file_exists(APPPATH.'controllers/'.$this->fetch_directory().$segments[0].EXT))
				{
					return $this->error_404();
				}
			}
			else
			{
				$this->set_class($this->default_controller);
				$this->set_method('index');

				// Does the default controller exist in the sub-folder?
				if ( ! file_exists(APPPATH.'controllers/'.$this->fetch_directory().$this->default_controller.EXT))
				{
					$this->directory = '';
					return array();
				}
			}

			return $segments;
		}

		// Can't find the requested controller...
		return $this->error_404();
	}

	function error_404()
	{
		$this->directory = "";
		$segments = array();
		$segments[] = $this->error_controller;
		$segments[] = $this->error_method_404;
		return $segments;
	}

	function fetch_class()
	{
		// if method doesn't exist in class, change
		// class to error and method to error_404
		$this->check_method();

		return $this->class;
	}

	function check_method()
	{
		$ignore_remap = true;

		$class = $this->class;
		if (class_exists($class))
		{
			// methods for this class
			$class_methods = array_map('strtolower', get_class_methods($class));

			// ignore controllers using _remap()
			if($ignore_remap && in_array('_remap', $class_methods))
			{
				return;
			}

			if (! in_array(strtolower($this->method), $class_methods))
			{
				$this->directory = "";
				$this->class = $this->error_controller;
				$this->method = $this->error_method_404;
				include(APPPATH.'controllers/'.$this->fetch_directory().$this->error_controller.EXT);
			}
		}
	}

	function show_404()
	{
		include(APPPATH.'controllers/'.$this->fetch_directory().$this->error_controller.EXT);
		call_user_func(array($this->error_controller, $this->error_method_404));
	}

}
?>

这个是控制器

<?php  if (!defined('BASEPATH')) exit('No direct script access allowed');
include_once("base.php");

class Error extends Controller {
	function error_404()
	{
		$this->load->view($this->_getTheme().'error_404_view', $this->data);
	}
}
?>