.htaccess blocking OScommerce login

I have a site where oscommerce is installed in a directory called /shop. I recently installed WordPress in the directory /blog and then followed the WordPress procedure to move the index.php file to the root so that the blog is what comes up by default. The problem is that something in the .htaccess file in the root is not allowing me to get to /shop/admin – I just get the WordPress 404 error. I’m guessing it’s because both use index.php, but I haven’t yet been able to determine how to modify my .htaccess file to get it to ignore the /shop/admin. Can anyone help?