Sandy bought a used computer from a guy, and it's filled with software. Is that legit? Leo says that chances are, the computer hardware is fine, but it will be filled with pirated software that she doesn't have a right to or paid for. And chances are, it'll be a timed trial that will stop working down the road. That's a drag, especially if the timed software is Windows itself. But by then, the guy is gone. And he may also have hidden key loggers and stuff.
Sandy should format the hard drive and install Windows 10 back on it and start from scratch. To make sure it's authenticated, she should type Windows Key plus X, select "system", and see if it says "Windows 10 trial." If it does, then she's going to run out of time and it'll stop working. She would have to buy Windows 10 and reinstall it. If it doesn't, then she owns Windows with that computer and she can just format the drive and reinstall.
Sandy should Google "Windows Media Creation Tool", and then she can download a copy of Windows and put it on a USB key to install it.