How to do fresh install of Mountain Lion?

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?

1. Back up your data.

2. Boot into the Recovery System by holding ⌘+R during early boot.

3. Select Disk Utility and format your drive where OS X and your files were stored on.

4. Quit Disk Utility and select "Install Mountain Lion".

You should make a boot disk first, then you can boot from it and install.

Make a boot disk

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.

1. First off you'll need to redownload the Mountain Lion app from the app store, unfortunately it disappears after you install
2. Right click on “Install Mac OS X Mountain Lion.app” and choose the option to “Show Package Contents.”
3. Inside the Contents folder that appears you will find a SharedSupport folder and inside the SharedSupport folder you will find the “InstallESD.dmg.” This is the Mountain Lion boot disc image you are looking for.
4. Copy “InstallESD.dmg” to another folder like the Desktop.
5. Plug in your USB drive of at least 8 gigs and Launch Disk Utility from the Utilities folder in your apps.
6. Select the drive in the left hand pane and click on the "erase" tab Set the format to MAC OS Journaled and give it a name like Mountain Lion OSX or something you'll remember. Click Erase
7. Click on the restore tab, in the source select the InstallESD.dmg file we copied earlier, in the destination tab select your USB drive. Click Restore

Clean Install from boot disk

1. Reboot your system and hold alt/option key as soon as your machine powers up. When you see a picture of your bootdisk, click on it to boot from it.
2. Once booted from the Mountain Lion installation media you will have the opportunity to run Disk Utility and erase your Mac’s primary hard drive. From the Erase tab select your Mac’s primary hard drive and choose “Mac OS Extended (Journaled)” as the format. Before you erase the drive confirm you have a known good backup then click Erase.
3. With your Mac’s primary hard drive wiped clean it is now time to begin installing Mountain Lion. Quit Disk Utility to go back to the Mountain Lion installer. Choose your Mac’s primary hard drive as the destination, and continue.

Voila, clean mac.