Angels have the Phone Box
Really, at this point, I don't think it's necessary. Unless you find any ransom notes in any of your data folders:
The Locky Ransomware Encrypts Local Files and Unmapped Network Shares
But if you're up to a clean install, that's always a good thing.
I think you caught it in time. Not many people monitor their CPU usage and processes like you and I do. Those who do, see things as soon as they start to happen. Trojan downloaders need time to phone home, to find a site that's not been shutdown, wait for instructions, download the payload, and then execute. Mind you, all this can happen in a flash, but sometimes we get lucky, and we stop them in their tracks. Defender certainly did its job for you this time.
It's likely this came in as an email attachment from a phishing email. If you use an email client (Outlook/Thunderbird, WindowsLiveMail, etc.), and have it sent to auto-preview messages, simply previewing a message can be enough to trigger the trojan. Other times you actually have to try to open the attachment for it to start downloading junk.
If you can, I would use best practice for backups, the 3-2-1 method: 3 rotating backups, 2 taking turns being connected to the machine, and one off-site. The best backup method I have found is Macrium Reflect Free. It can be set to run automatically; images can be mounted and single files pulled off if needed. You can even automatically add Macrium to your boot menu. Plus if your hard drive bites the dust, a new drive can be imaged and you're back in business within a short amount of time. No installing of programs necessary.
I will make a few suggestions for your computer security, if it's okay:
You see that this (and most downloaders) download their payload(s) to the appdata/temp file directory and attempt to execute from there, so a program which prevents executable files from executing out of uncommon areas such as these would help. The one I use is:
CryptoPrevent (free version)
Firefox browser, with appropriate security settings in place (I can go into that in another post).
Set your email client so it doesn't auto-preview, and never open attachments you are not expecting.
Defender is good, and certainly saved your bacon this time. ESET NOD32 (paid) would be a step up, and you can find it a lot on sale at Newegg. They also have a 30-day trial if you want to test it out. It's one of my favs.
Malwarebytes Antimalware: Free is good, but it's passive. If you can swing it, the Pro (paid) version is active protection, and their beta anti-ransomware module will be rolled into the Pro version as soon as it's out of beta.
SuperAntiSpyware Free: another passive one, clearing tracking cookies and some malware.
MBAE Malwarebytes Antiexploit: free version provides protection for exploits against your browsers. The paid version provides protection for all internet-facing applications on the computer.
Unchecky: prevent those unwanted PUPs and PUMs from installing along with other software.
A layered approach is required, as each program has its niche/specialty.