You'll need a few USB thumb drives or a USB HD unit to start with.
- Backup your data and Apps if you didn't download them from the Apps store.
- Download from the Apps store the OS installer for your system.
- Make a copy of the installer in a temp folder (just in case) Using the OS installer prep up a USB drive as a bootable disk. Make sure afterwards it is able to boot up your system (Use the Option key during restart to select it). Once its working copy to it the OS installer. This is now your recovery disk.
Now restart the system using your recovery USB thumb drive and run the OS installer app make sure you select your internal disk reformat the drive and re-install the OS.
Restart the system again without the thumb drive, copy back anything you want - Done!
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