My site is being hacked weekly – can someone please help?

Hi all.

Between every half a week to a week, the file
“wp-blog-header.php”
in my blog:
http://www.danceinisrael.com
is being changed into something else.
I am using WP 2.8.2.
I changed all my FTP passwords.
My home directory is set to permission of 555

I find out this is the case using this plugin:
http://wordpress.org/extend/plugins/wordpress-file-monitor/

I don’t have any unusual plugins running on the website.

And still, for over two months, this website is being hacked. and I have no clue how to find the breach.

Any one willing to help ?