it will be in your purchased tab in the Mac app store download again but this times once its downloaded to run it just quit it if it auto opens.
than use this small app and a 8GB usb stick to make a bootable copy of ML
now once the usb is done, plug it in and restart your mac holding the option key when you hear the chime you will see the usb to boot from, now you can use disk utility to do a clean install
Hello All,I would also like to know about the clean install? Do you need to use a USB or what? I already have a USB with ML already on it, so could I use it on my iMac and hold the option key after I hear the chimes? I would like to know so I can try it too. If you know, Please let me know...Thank You.
Insert the USB stick and start the creator application.
When complete (it takes a while with USB 2) You can eject the USB stick and you now have a bootable image,
Start your MAc whilst holding down the "Option" key
If you wish to, you can erase the drive to start afresh, or perform an (EFI) over the top installation of ML
Over the top will retain all your Apps, a clean install will require you to download and re-install them from the App store. If you have third party apps, make a note of them prior to the Installation