You can do a restore if you can get into Windows, just type "reset" in the start menu and something along the lines of "Reset this PC" should come up. Then you can select a complete restore from within the options in windows 10 you can choose to keep the users files or delete them. I would try keeping the first but if you do it and it's still laggy, move her stuff to a external drive and do a complete reinstall, as for the key being in the bios if you want to be safe go to the control panel>system and security>system then at the bottom the computer key should be displayed under "windows activation"