Lion and Mountain Lion (if pre-installed on the computer at purchase*) Note: You will need an active Internet connection. I suggest using Ethernet if possible because it is three times faster than wireless. 1. Restart the computer while holding down the COMMAND and R keys until the Mac OS X Utilities window appears. 2. Select Disk Utility from the Mac OS X Utilities window and click on the Continue button. 3. After DU loads select your startup volume (usually Macintosh HD) from the left side list. Click on the Erase tab in the DU main window. 4. Set the format type to Mac OS Extended (Journaled.) Optionally, click on the Security button and set the Zero Data option to one-pass. 5. Click on the Erase button and wait until the process has completed. 6. Quit DU and return to the Mac OS X Utilities window. 7. Select Reinstall Lion/Mountain Lion and click on the Install button. 8. Upon completion shutdown the computer.
These instructions can be found
HERE.