I’ve done this several times to myself now, but if you have just installed Drupal and then try to access the /user URI and get a “404 Not Found” error, fear not, you’re probably just missing the Drupal .htaccess file in the root directory of your installation.
To solve the problem, copy a .htaccess file from a similar installation or a Drupal download file to the root directory of your new installation. Just make sure you get a .htaccess file that corresponds to the Drupal version you’re installing. (Get a Drupal 6.38 .htaccess file for a Drupal 6.38 installation.)
Always happens when I copy a Drupal website
I keep doing this to myself when I copy a production Drupal website onto my laptop for testing and updating. In production I don't always keep the .htaccess file, I sometimes copy that text into my main Apache configuration files, and as a result, when I copy a production site to my laptop for testing and updating, I don’t have a .htaccess file and then run into this problem (which is usually followed by a few moments of panic).
A good way to tell if this is your problem is to access the ?q=user/login URI on your website. By this I mean that if you’re working on a website you’ve named foo.com, the full URL would be http://foo.com/?q=user/login. If that works, this is almost certainly your problem.
(For other possible problems when copying Drupal websites, don’t forget to check your sites/default/settings.php file.)