Restoring Lost OS X Files
If you erase or otherwise "lose" an OS X system application or utility (such as Mail or Disk Utility, for example) you can restore them without having to reinstall OS X.
Download the shareware utility
Pacifist. Read the documentation. Use Pacifist to extract a fresh copy of the missing application from the file archives on your OS X Installer disc. The file archives are usually found on the installer disc in the /System/Installations/ folder.
After restoring the application you need to reinstall the OS X Combo Updater for the version of OS X you currently have installed. This assures the new copy of the application is updated to the current version.
Lastly, it's a good idea to repair permissions.
