Enigma1 :
How do i get rid of the old OS safely?
It's not unsafe to leave it there. The old OS doesn't suddenly "wake up" and ring havoc!
You are over thinking, the old OS is just JUNK that's taking up space, so as long as you don't care about the garbage u can just leave it there. 2 options.
1. Move your data to "somewhere else" off this HD, then do a FORMAT HD (completely obliterate it clean), then move the data back from "somewhere else." This is the CLEAN way of doing it, but of course you need that "somewhere else" to assist.
2. Leave it alone, or just delete the WINDOWS and the PROGRAM FILES directories will rid of 90% of the stuff. The USER directory, you *may* have stuff here that you want to save, this directory, you may want to think about, and/or scan it, and/or burn it to a BR just in case some days u ask, do I have a copy of...?
Do I have to install a program to do that?
No. Just use Windows built-in tools.