1. Download and unzip a fresh copy of the master Zen Cart zip.

2. Compare your server's list of filenames with the ones from the original fileset, specifically for these folders:
-/includes/languages/english/modules/payment
-/includes/modules/payment

3. Delete any that aren't in the master set and which you haven't intentionally installed.

4. Re-upload all of the files in those folders from the master fileset, making sure all files load without any errors.

5. Re-apply the PayPal patch, making sure to put the right files in the right folders, as found in the patch zip. You may as well use the newest patch while you're at it.