Start fresh. If your current OS disk is smaller than 50 GB, you can try cloning it, but I assume yours is bigger than than and that means you have huge amounts of garbage on it. Only a fresh install will get rid of registry and user profile bloating.
BTW. A 1 TB drive for OS & programs is a waste of space. You would be better off with a 128 GB Samsung 850 Pro SSD. 128 GB is more than enough for an OS disk.
I've found a 120GB SSD too small when we have installed all the programs a filmmaker or VFX artist might need. A 256GB SSD should be more than big enough though.
When I upgrade I'll be doing a full reinstall of all programs and OS for the reasons cc_merchant mentioned.
Fun times ahead.
Thanks all. I did a clean windows install. I already had my new 1 TB drive. So I used it. The next time one if my 1 TB raid drive dies I'll swap it out and use a 256 SSD drive for my OS.