Delete the .pub key and it's contents, be careful not to delete anything else. Produce a registry backup first if in doubt.

Next go to "HKEY_CLASSES_ROOT\.pub" and delete it - just the .pub key nothing else. (If you look at it you should see it is a "pub_auto_file".). Exit regedit when you're done and reboot.

The next time you double click a .pub file it should ask you which program you want to open it with, you can cancel this as you don't have publisher installed.