I know what you mean by having bad knees!! I too find that most exercises are tough on the knees - expecially afterwards. I can walk for about 20 - 25 min., but if I do anymore I can't sleep all night. Jumping up and down to a video would be a two or three day pain filled experience.
I have been trying to do my upper arms by holding on the arms of my chair and lowering and raising myself. It at least helps a bit!!!
I have been thinking of getting one of those big balls and using that to do some things. I watched someone on one and they did situps and a few other things that I thought I might be able to handle.. I would need to get one of the bigger ones, though.
It looks like you have been doing very well with losing.
Keep up the good fight!!!!