Help! Site might have been Hacked.

[Simon Lloyd]

You will have an unsecure script possibly from a modification or third party software or your server is insecure, maybe shared hosting environment?

You'll need to check your files and folders for a file that shouldn't belong there or check when core files were last modified as you may have had some code injected. If you haven't edited your core files you should upload a fresh set via ftp in ascii mode with overwrite, you do not need to upload images and make sure that /install/install.php is deleted before upload, alos make any edits needed to /includes/config.php before upload for your server requirements.

After that it's a matter of finding out how they got in, through which addon or mod or through the server, you'll need to contact your server people to check the logs if your dont have your own dedicated server.