atal error: Call to undefined function wp_get_current_user()

HI!
I have a new 3.1 wordpress site,but have some difficulties few weeks ago
Actually is looking like this:”Fatal error: Call to undefined function wp_get_current_user() in D:\Program Files\xampp\htdocs\wp-includes\user.php on line 254″ after i want install a new plugin,or remove a plugin.
its which already works in other websites. whit the lates werson of word press
Some able to help me fix this bug?

Im read a forums and im see,that someone else have a same problem,and hi get the answear “I think its actually a bug in WordPress – the solution is to add a require_once call to the top of capabilities.php. “
but im not find enything,how can i do this

Please,Help me!
Thanks Zoltan