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?
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.