UPDATE: WordPress 2.6.5 has been released. This is still no legitimate version 2.6.4 and there never will be.
As of the time of this post, there is no such thing as WordPress 2.6.4. There also likely never will be as 2.7 will be the next version of WordPress assuming no emergency security fixes are needed before it comes out.
If you “upgraded” to “2.6.4″, then you have installed a fake trojan version (full description is here and here), you should delete your
wp-includes folders and replace them with fresh copies downloaded from WordPress.org, the official site. If you have a recent database backup from before you upgraded, I would also advise dumping your database and using that backup just to be safe. You should also of course change all of your WordPress user account passwords.
Please be smart when it comes to WordPress! Because it’s so immensely popular, it has become the target of spoofers and hackers. Always type in the URL to the official site (rather than using a link and not paying attention) if you are trying to download the latest version.