Mac OS X 10.6: Restoring your computer’s software
If you delete some Apple software or have software problems you can’t fix, you can restore some or all of the software on your computer to its original state. Restoring software replaces your Mac OS or application software with an exact copy of the original software that came with your computer.
To restore the Apple software that came with your computer to its original state, use the Software Restore or installation disc or discs that came with your computer.
Important:Before you begin, back up the files for each user account on your computer. Depending on the computer and restore process, some information may be lost.
Click the link for detailed instructions.