My Mac is very slow.
My Mac is running OS X Mountain Lion after an upgrade from Lion.
How can I do a fresh install the OSX Mountain Lion, with a clean HDD (all of my documents deleted), so it feels like a new MacBook?
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Back up your data.
Boot into the Recovery System by holding ⌘+R during early boot.
Select Disk Utility and format your drive where OS X and your files were stored on.
Quit Disk Utility and select "Install Mountain Lion".
You should make a boot disk first, then you can boot from it and install.
Before we start you should get your hands on a usb drive with at least 8 gigs of space that you're happy to wipe clean.
Voila, clean mac.
How much RAM do you have. For Mountain Lion , you need 8 GB RAM. Other thing, I would like to suggest you to clean your Mac, remove all system junks from your Mac and gain more free spaces. Still, if your Mac is slow then do fresh installation.