Do u have the original install disc? Reload OS from scratch.
If u don't, and I assume you have no original nothing on-hand since u asked, u will have to download ML from Apple, then create a bootable USB HD or thumb drive with it (8G minimum), boot with that (USB) then run disk util and erase everything (with partition) on the Internal drive, THEN re-install OS from scratch from the USB HD/thumbdrive.
Here's an
excellent article on how to do that.