I've posted before that I'm a big Cathe Friedrich fan. She has a Low Impact Series (but high intensity) that would be easier on your knees. She is equipment-heavy, though, so you may need to invest in a few pieces. I have all of these except for the cycle DVD and that's because I don't have a spin bike. Good luck.
http://shop.cathe.com/workout_routines_shock_cardio_s/94.htm