IF you have Ultimate SEO URLs installed, or any kind of URL Rewriting going on, AND you have cPanel as your hosting control panel, then take a look at my post here:

http://www.zen-cart.com/forum/showth...841#post986841

This solves a problem which occurs with SEO URL Rewriitng when using cPanel as the hosting control panel - you try to login to 'admin' and get redirected to "404 - Page Not Found".

Vger