Angels have the Phone Box
So sorry to hear you're having problems again.
I'll make one scan suggestion, then, if anything is found, I think the clean install is the way to go.
SOPHOS Virus Scanner
If either finds anything, then we possibly didn't get things completely cleared out, (even though Slartybart confirmed an all-clear), or you got infected again somehow. And, yes, some infections are ticking time bombs, lying dormant for a while. I'm surprised though, to think we didn't get everything...we were quite thorough.
In the case of a clean install, the following procedure will absolutely ensure nothing remains:
Diskpart “Clean All”
This writes 0s and 1s to every sector of the drive. It will kill everything. Doing it this way will kill any infections that can survive a format; it will also get rid of any hidden partitions created by malware/viruses. It also can take quite a while.
Disconnect all drives except the one you are installing to, so you will have only 1 drive connected to the system. It will be designated as drive 0.
Boot into the Windows installer.
The first screen is select your language, don't do it.
Hold down the shift key and press F10.
You will get a command prompt.
Type the following commands one at a time and hit enter after each, and wait for a response before typing the next command:
You will now be back at the select your language screen.
select disk 0
Continue the install as usual.
When you get to where do you want to install windows the drive will show as unallocated.
Press the new button and the installer will create the partitions.
Then click next and the install will begin. Be sure you're installing the correct version (Home or Pro), and don't enter a key if requested.
The clean all will take a long time. (A 1TB drive will take 4-6 hours.) Just let it do its thing; it will tell you when it is finished.