security fault with WordPress 3.3.1

The code below keeps getting put on my site, I have checked every php page and it has no code, locked all passwords down and the only way I can get rid of it, is by installing word press again WordPress 3.3.1.
I have now installed this version 5 times and this code is clear then comes back ??

[Code moderated. Please do not post hack code blocks in the forums. Please use the pastebin]