Could be that you got hacked. You an always try replacing the entire "administrator" directory with a freshly downloaded file set. If you don't have any customizations there, you should be fine.
can you direct me to something to tell me how to replace the administrator directory? We have two: one for the old installation and one for the new. Let me check to see if that script is trying to run on the old....
When I say overwrite the Admin directory, I just mean delete the old admin files and upload the new ones through FTP...from the same Joomla version of course.
But using a backup will be better.
After you revert to the backup and test it, make sure to upgrade to the most recent versions of Joomla and all extensions to avoid it happening again.