Hello. I'm going to buy a used laptop today.
Now, the thing is, the current owner (whom I do not know personally) said he will do a clean install. If it'll actually appear to be a fresh Windows, should I just download a few different virus scanners to make sure it's fine, or should I reinstall Windows from scratch myself?
Not really sure how well the potential viruses could be hidden.
While you could run a risk of viruses, there's an extremely low likelihood of that. If anything, I'd be worried about potential incriminating evidence being on the computer. That's even less likely, but still...
What I always recommend for every computer purchased from a non major name distributor is a clean install from a professional (unless you know how to securely wipe a hard drive). That's what I always recommend, since you never know what may be on the computer you received. And no, formatting is never enough for real potential threats.
Either way you look at it, the best method of "cleaning" a used computer is always the same answer. Wipe it, install a clean OS you purchased, and then run/install whatever you wish on it.
If you buy a used computer, always install a clean copy of an OS. It's better that you run your own key in it, just to help prevent some bullshit from microsoft, should a key expire, or the copy is a little messed up on the backend.
Clean installs are the only real good idea for used computers. I'd say the same with used phones, if you know what you're doing.