There are lots of low impact aerobics videos available online. Before I joined a gym, that was my primary form of exercise
If you check out www.collagevideo.com
they have scads there with video clips as well as reviews.
I've had bad knees since I was 14 and a bad ankle since I trashed it when I was 25. Things really aren't any worse than they were since they first started, but any weight you lose is going to help the weak joints. I can do a lot more now that I am 80 pounds less!, but alas I still cannot tolerate jumping around and running and probably never will be able to.