HACK: eval-gzinflate base64_decode

Hello,

The eval-gzinflate base64_decode hack has attacked my WordPress-blogg. I found a way to remove it – by simply swopping all files with my backup files. However, after some minutes, the malicious code always returns.

I have removed almost all plugins from the server but I can’t find its access door. The FTP password is changed as well. Can the code have reached my database perhaps? I’m not very good with phpmyadmin but is it something I should do there to prevent the code from coming back?

Any help is much appreciated!