base_url() will echo:
http://localhost/project
where as site_url() will echo:
http://localhost/project/index.php
You want to get to http://localhost/project/index.php/aboutus but with base_url() you're only getting to http://localhost/project/aboutus which is giving you the error.
You can do two things,
this:
<a href="<?php echo base_url('index.php/aboutus'); ?>">AboutUs</a>
which means adding the index.php before aboutus
or this:
<a href="<?php echo site_url('aboutus'); ?>">AboutUs</a>
which means changing base_url() to site_url().
Make sure that you are loading the helper in the controller:
$this->load->helper(url);
Or in application/config/autoload.php go to the line which says:
$autoload['helper'] = array();
and do this:
$autoload['helper'] = array('url');
and it will be included in every controller you now have.
If you have short tags enabled you can write your a tag like this:
<a href="<?=site_url('aboutus');?>">About Us</a>
or if you have the url helper you can write it like this:
echo anchor('aboutus', 'About Us', 'title="About Us"');