Clean install. Pretty easy, you just have to prepare for it:
1. Documentation - Find all of your username/passwords for sites you visit. Login details for any websites. Serial numbers for any licensed software. Email logins, etc.
The Windows license key!
You'll need 2 USB sticks.
2. 1 for the OS, if you don't have a DVD for it already. Create your own Win 10 install here:
https://www.microsoft.com/en-us/software-download/windows10
Another USB stick for all the drivers for your hardware. Go to the manufacturers websites, and find those now, and save them to this USB stick.
3. Find and save all the install files for whatever applications you are going to install later.
Find your SteamApps folder and copy it off to a different drive. We'll use this later.
4. Your personal data. Find and save ALL of that off to your other drive. Including game saves if you want.
5. Check all that stuff again, just to be sure.
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6. The install
Disconnect ALL drives except the new SSD
Power up and boot from your OS install media
Follow all the prompts.
Eventually, it will finish.
Assuming the OS installed a LAN driver, you should be connected online. If it did not, then you install it from the USB you created in step 2.
7. Now let it run all of its updates.
Verify the activation status.
8. Once it is all updated, now we can power off and reconnect the other drive(s)
9. Now you start to install all your applications.
With the new Steam client you're installing, you can simply tell it where the SteamApps folder is. That can live on a different drive, and the 120GB SSD you have is too small for games.
Tell it like this:
Steam games location
In the steam client:
Steam
Settings
Downloads
Steam Library Folders
Add library folder
10 Have fun.