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#21
As you say, you don't know what is on the system. Another reason for cleaning everything up, it is very possible that the previous owner has installed software to track what you're doing to steal at the right moment. This is a dirty example that actually happens often in regards to credit card fraud or exploiting the system to install ransomware.
It is strongly recommended to backup all personal files you don't want to loose, then wipe every drive (or replace) and clean install Windows.
What many do when they get a used machine, wipe the drive and install Windows all over again, for above reason.