How To Fresh Install ML

A clean reinstall of Mountain Lion involves creating a bootable USB stick from the installer downloaded from the App Store. You would then boot from it by holding ALT (Option) while rebooting. When you boot into the USB stick, use Disk Utility to delete both the Lion and Windows partitions from your drive.

As for clean reinstalling Lion, since it came pre-installed, you would have to backup your hard drive and 'erase' it which would then prompt you to reinstall the OS.

Or, if you have working (bootable) OS X on external (USB) Hdd, simply:

• Time Machine!!! (Don't forget backup) :)
• download ML from Appstore
• after the download it start the installation
• quit it
• move the downloaded mountain installation app from /Applications into the external HDD
• reboot mac
• hold down ALT and boot the external USB
• if want you can repartition the internal HDD with Disk utility.app (remember, you're booted from external HDD, so, want repartition the internal one...)
• run the copied installation app
• select your internal HDD as target
• install
• done

I'we done installation this way because i'm changed my HDD into SSD, so I used my old internal HDD as bootable external HDD...