I'd speak to the boss first and ask him to please clarify this with her because she is misunderstanding and also misrepresenting your involvement and it is very concerning for the success of the project she is working on. And also let him know about the personal calls, the lack of attention in general to her work, as they need to be aware of this. I would ask the boss to call a meeting with both you, her, and the boss, (or clear it with them to have a one-on-one official meeting with her to discuss expectations and parameters so you can act more manager-ish) so you can explain that you are only working on "X" project with her to show how the excel files should work, but you are not checking it for her and not fixing her errors; that she is 100% responsible for checking over her results to make sure they are correct after you advise her of your results. And that any errors are completely on her, so she needs to be much more vigilant in getting things corrected. That way, you have confirmation that she's been told plainly what the expectations are, and you can get the boss to clarify your role in all of this.
As far as her following you or discussing her personal problems... you need to develop a polite spine. It's basically telling her nicely "Hey, coworker, I'm sorry but I can't talk right now as I have something I have to work on." and then turn away and work. If she continues talking, then something like "Really, coworker, I'm sorry but I need to get this done/don't have the time right now. We'll talk another time." and get back to work. She might think this is rude... but do it with a smile and not any snark in the voice each and every time she tries to discuss personal whine stuff and she'll eventually leave you alone. If it's work related cool, if not, then "don't have the time right now..."
Honestly, it sounds like you and the boss are setting her up to fail, tho. Is that what is happening? Because if she can't do the job at all, then she should be moved to another position or let go if she is incompetent.
Side note: There are plenty of 40 somethings (and 50 and 60...) that can do highly technical/complicated stuff. And companies do still hire 40/50 year olds all the time, so it isn't an indication of how long she's been with your company. Don't be ageist; that's going to make you come across as petty and ignorant. I don't think you are, but you definitely seem to be coming across as prejudiced, and likely didn't even realize it. You should be better than that if you're enlightened enough to be hanging out here. ;)