can’t upgrade from 2.02 to 2.03

I just upgraded from 2.02 and for that, I replaced the old files by the new ones. but now, here’s what i’m getting where ever i try to reach on my blog:
Fatal error: Call to undefined function: wp_get_current_user() in /home/www/sork.freehostia.com/wp-includes/functions.php on line 311

I had persian version. i even deleted everything and put back my old 2.02 filesm but that didn work neither. still the same error, but on a different line. what should i do?