You have had your core files compromised, you've had a virus or such like, check your php files for BASE64 code, start with newthread.php, if you have been compromised you need to think why? insecure scripts or ones that haven't been updated are usually the culprit.
To rectify the issue of corrupt files simply upload a whole new batch of core files (with overwrite) via ftp, you can leave out the images folder and delete /install/install.php before uploading.
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