Hi Karen,
I have yucky knees, too. I have really noticed much less pain by removing the step from most of my cardio workouts. I do one step workout a week (usually an interval one, Imax or Intense Moves), and the rest of my cardio I do on the floor, even if the workout is a step one.
I delete almost all the hight impact from all my workouts--just substitute knee-up lunge back for power scissors. I use VERY large arm movements, and REALLY use my space to make up for the lack of intensity.
As for strength, I have found that by keeping my legs VERY wide during squats (almost to the point of being a plie squat), that my knees do not hurt. Also, I keep most lunges static, even if Cathe is doing forward or back lunges. Form is critical!!
Everyone's knee things are different. I'm more bothered by squats, but others feel twinges with lunges...The most important thing is to STOP if you feel pain!!
Others may also reccomend floor work to sub for standing leg work. The floor work in Karen Voight's Great Weighted Workout is OK. PS legs floor work is wonderful. Gettin the quads is tough for me without doing lunges and squats, and I have been able to put them back in by not doing step workouts as often...
Be extremely careful if you are working out on carpet. I'm hoping to get some great Mats to help with this.
Hope some of this helped...
Wendy