If Grub sees another install(or remnants thereof), it'll add it to the boot menu, so you'll still see it, even if you boot from a computer that never had that O/S installed. You can update Grub from the blank computer to get rid of that entry
There's numerous ways to rescue a system. As with most things, if they break, they can be unbroken, but the variables are how much skill and time it will take to do so. If you don't have much in the way of customization, I'd backup /home and maybe /etc, then do a reinstall if things get too out of hand. That's probably quicker, but if you want to learn how you screwed it up, and want to spend the time, outside of a major disaster you should be able to use the existing system.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore, all progress depends on the unreasonable man.