Custom 404.php page not working

In WP 2.0 my custom made 404.php page (in my theme’s directory) is not working.
My .htacces looks like this:
RewriteEngine on

# BEGIN WordPress
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php
ErrorDocument 404 /index.php?error=404
</IfModule>

# END WordPress

When you type an invalid url, example:
http://vitorazevedo.com/cocacola
you get this:
Not Found
The requested URL /php-bin/index.php was not found on this server.

Additionally, a 500 Internal Server Error error was encountered while trying to use an ErrorDocument to handle the request.

Any help?