Step-by-step to re-create your USB installer, in the hope that it will fix the problem:
1. Grab your USB drive and go to a working Mac
2. Turn on the Mac, then plug it in
3. Open the Mac App Store, login to the account you used to purchase ML
4. Download ML then close the installer when it opens
5. Now follow
all of these instructions (except downloading ML...you've just done that) and then come back to this page
6. Once you've made the USB installer, remove it from the Mac - make sure you eject the USB drive by right-clicking and clicking 'eject', or dragging it to the trash can before you unplug it
7. Now go to your MacBook Pro, plug in the USB
8. Turn the Mac on and hold down 'option', or 'alt', then select the USB drive to boot into
9. Run disk utility before doing the install - completely erase your Mac's hard drive and format to Mac OS X Extended (Journaled)
10. Now install on the new Macintosh HD partition (or whatever you called it)