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To restore the Mac Pro to factory settings:
1. Insert the Mac OS X DVD that came with your Mac, and press the C key while your Mac is starting.
2. Go to Utilities menu > Disk Utility, select Macintosh HD on the sidebar, go to Erase tab and erase the disk.
3. Close Disk Utility and reinstall Mac OS X, so it will be installed using default settings
This will erase everything, so make a backup of your data before