A clean install is just that - it wipes out everything.
You can't do an "in place" clean install (as far as I'm aware). Even if developers wanted to provide it it would be a nightmare. I'm not just blowing it out of proportion (I'm a developer) - it would be a nightmare.
PRIOR TO ANY OF THIS MAKE A FULL SYSTEM BACKUP. Hard drives are cheap. If you can't afford a hard drive for backup, then DO NOT ATTEMPT THIS - NO SERIOUSLY. AND WHEN I SAY SERIOUSLY I MEAN SERIOUSLY SERIOUSLY.
That said, you CAN do a reasonable clean install and still maintain MOST of your information. Here's how:
Create a bootable DVD, flash drive or hard drive with the ML installer.
1a. Make sure it really is bootable!
- Back up your personal data. (and any other users)
- Back up your applications.
- DO NOT BACK UP system info (this is the downside). Note that some applications put info in weird or unusual places - you may have to reinstall that software.
- Reboot the system using the DVD, flash drive or hard drive.
- Run the ML installer.
- Copy all the user data and applications back.
- Determine what no longer works correctly and reinstall those applications (yes - seriously NOT fun).
I did this with Lion (but not ML) but AFAIK it should work.
Clean installs "break" applications (many install libraries to work correctly or create 'hidden' directories).
Good luck (and remember: FULL SYSTEM BACKUP FIRST)