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.