Hopefully, it was just the installation file that lurked behind - presumably it would only be harmful if you clicked on it and told it install CCleaner again.

Hopefully.

At risk of sounding like a stuck record (remember those?), have you considered using an alternative operating system? None is invulnerable to attack and I'd imagine they're all capable of being thoroughly annoying. However, Windows does attract an awful lot of malware. Presumably this is the result of Microsoft's dominance, but I can't help wondering whether the fact that so many attacks succeed might just have something to do with vulnerabilities in Windows.

W10 is doubtless very secure, but I can't bring myself to trust it - all that business with the telemetry (and everything else the default privacy settings allow to be sent to Microsoft), and the updates which remain out of the user's control.

I'm very happy with Linux (specifically, Linux Mint) but for those who prefer the "reassuringly expensive" approach, there's always Apple. Not for me (seems too keen on vendor lock-in), but it just seems more secure than Windows and at least slightly less of a privacy threat.

If you're adventurous, there are options like FreeBSD - which looks a bit beyond me, but is very highly regarded.