if she works in the field then she should be well aware of the phrase
"Caveat Empor" (buyer beware).
You showed her a working machine, that had nothing wrong with it, she inspected the machine and agreed to buy it based on what it was worth. there is not a lot that she can do with that.
If she emails you again saying that she works in the field threatening you, tell her that you reasonably feel that you have done everything that is reasonable.
you've sold her a working laptop that she has inspected and seen working, that you've continued to communicate with her after the sale offering free technical advice, you've suffered her threats of court action and possible damage to your name as she threatens to take you to court, and possible psychological abuse as she threatens to arrive at your house.
Ask her to kindly take the laptop to a shop, where she may get it fixed, tell her that it's not your problem if she's dropped the machine, spilled something on it or otherwise broken it, it was sold as seen and that you've already gone above and beyond what you'd have expected to do as someone selling second hand items in a classified column.
She agreed to buy a working laptop that was working at the time of purchase, and to the best of your knowledge would have kept on working.
Don't tell her to bring it on, getting aggressive won't get you anywhere.
Don't tell her to go ahead and try and sue you, and that she'll loose as you sold in good faith and she later broke the machine, don't tell her that she'll loose and you'll counter claim for costs and damages, (cause that's just threatening her).
do ask her politely to stop contacting you.
If she doesn't stop contacting you then I suggest that you go to the police and explain the situation and ask if they can place a restraining order on her to stop her contacting you, take evidence of her threats in the form of the emails that she's sent you.
(don't tell her you're going to do this, cause again that's a threat).