[Simon Lloyd]

It seems to me that one or more of your core files hasn't been overwritten, you will also have a file or two which doesn't belong in your forum root which is rewriting the infection every time it doesn't see it, my suggestion would be to rename your forum folder add a new folder then name it to what your forum folder was, upload all fresh files (with the install/install.php deleted and the config.php.new edited for your database and renamed to config.php) and then try to access, if you can then you need to search your old folder for files that shouldn't be there, delete them, then upload with overwrite via ftp in ascii mode your fresh files in to the renamed folder, rename the temp folder to something else and then rename your old folder back to it's original and see how you go.