MacOSX Lion includes a recovery partition allowing you to easily erase all data on your Mac and reinstall your OS.
- keep your Mac powered, during the whole procedure
- your Mac must be connected to the internet
The procedure is as follows:
- Turn your Mac on, while holding the CMD + R keys down
- Select "Disk Utility" and click on Continue.
- Select the startup disk and go to the Erase Tab.
- Select the Mac OS Extended (Journaled), give a name to your disk and click on Erase.
- Disk Utility > Quit Disk Utility.
- Re-install MacOSX: click on "Reinstall Mac OS X" > "Continue".
- Follow the on-screen instructions.