Gastoneciacci,
Welcome to LQ.
Boot from your USB installation drive as follows:
http://www.psychocats.net/ubuntu/resetpassword
In any event I would be inclined to copy and paste all personal data to an external drive and then use either the lighter
MATE or
Xfce version of Linux Mint 18.3, as opposed to Cinnamon.
Better still, since this is a netbook, it would be better to use something truly lightweight such as
antiX:
https://antixlinux.com/