If you have the CDSEO BASIC X-Cart module (NOT PRO) installed on your site and your product and category links are returning "File Not Found" errors, it could be that your .htaccess is not set up properly for the server where your site is hosted.
Check your .htaccess file in your store directory. If the CDSEO code includes directives such as "SetHandler", "Forcetype" or "AcceptPathInfo", try commenting out those lines and add these in their place:
RewriteRule ^product-directory-name/(.*)$ product.php [L]
RewriteRule ^category-directory-name/(.*)$ home.php [L]