try to steer the conversations to pleasantries so she does not dwell so much on the past...you can talk about things that make her happy....shopping the grandchildren, the weather, the pets...make plans for the spring, talk about the upcoming holidays, describe the play you saw, how well the kids are doing in school, anything.
ask her OPINION about what color looks best on you, what color you should paint the kitchen bathroom, the cat, (even if you are not going to paint, getting her to offer her advice will be helpful) funny antics the pets are doing, how weird the neighbors are.
she NEEDS to feel useful, even if you just ask her to fold your clothes or what kind of potatoes she thinks are best....get creative....you have to give her a sense of purpose and not disrespect her, because it IS a disease.
(I attended an Alzeimer's seminar 2 weeks ago)