How do I know if a hacker or virus is messing around on my laptop?

** scan Free and reliable?

I have already posted a question where I described that my HP laptop suddenly switched off with an error message and is now spinning around:

private files disappeared, apps can't open, everything I type on the taskbar is released from it again.

Now see in the Windows (C) folder under users a folder named:

defaultuser100000

If I want to open, I first have to confirm administratively to continue. Picture attached. Don't dare to open the folder haha, is my hesitation correct or can I open it without worry?

How / where can I do a kind of virus / user scan for free but reliably?

Is there something in the task manager that I can look out for / find signs of where the problem is?

(For example: idle process CPU: 99, or user name: DWM-2, CPU: 1, UAC-V: deactivated)

Windows activation? It's my private HP laptop, bought at Saturn.

You are definitely on the safe side if you start your search on the BSI website. Heise-online is also good. Or the comparisons at Spiegel / Stern etc.