Please feel free to ask me questions, or let me know if anyone disagrees with me so that I can correct my response/educate myself further. Alternatively, you can also look into installing chromeOS instead of windows since chromeOS is relatively more locked down as compared to windows, and if your dad doesn't do any techy stuff on his PC anyway, chromeOS should more than suffice. Make sure to install things like ublock origin and Firefox/edge to the system so that he doesn't click on any malware ads in the future which would infect his PC.

And once you've reinstalled windows, you could create a non-admin account for your dad that he could use. Your clients can observe all you do on their screen and take control whenever they want. UltraViewer is designed to support customers, partners remotely. If you do a fresh windows install, it would automatically get rid of all the previously installed malwares. UltraViewer helps you control your partner's computer to support them as if you were sitting in front of their screen. I know that you can use a non admin account and try clearing as much malware as possible, but there's always a concern about the anti-malware softwares not being able to deal with FUD crypt RATs (Fully Undetected encrypted Remote Access Trojans). I honestly would recommend you to do a fresh install of windows at this point.

Ultraviewer