Have you talked to your doctor about your knee pain ? If not, a person should try to check with their doctor. They might suggest surgery or maybe physical therapy. It depends on what is causing your knee pain. Because no one should be exercising with chronic pain.
What to do ? If your doctor has told you it's okay to exercise as long as it doesn't hurt, stick to low impact exercises. Walking is wonderful cardiovascular exercise. If you can walk without pain, take a daily walk. There are members who will tell you how they lost 100+ pounds just by walking and watching what they ate.
If walking for long periods causes pain, then walk for as long as you can. If that means you walk for 10 minutes, then walk for 10 minutes. You can break your workouts up into shorter segments and still get all the benefits. So, if you can't walk for 30 minutes, then take 3 ten minute walks spaced out through the day.
If you do find it difficult to stand for long periods, then how about some chair aerobics ? Coach Nicole has a cardio workout you can do while sitting in a chair. You can find more chair workouts posted on YOUTUBE. they even have chair workout DVDs on amazon. www.sparkpeople.com/resource/videos-detail
If you have a gym membership, does the gym have a pool ? If so, there is a class called aqua zumba. that might be something fun you could try that would be gentle on your knee. Swimming and water aerobics would be two options too. being in the water helps to take pressure off the joints.
Also, talk to your doctor about a recumbent cycle. That's a bike where you sit in a reclined position to peddle. You may find that less stressful on your knee too.
And well, there are always upper body strength workouts. Work your upper body muscles while your knee rests. Get a set of resistance bands and do upper body exercises while sitting in a chair.
Depending on how bad your knee is, a possible option for squats would be ball squats. You place a stability ball between your back and the wall. The ball supports your body as you do the squats. that too will take pressure off your knee. But, once again, since we don't know the state of your knee, this might hurt too.
In this video, they use hand weights, you don't have to use hand weights. www.sparkpeople.com/resource/exercises.asp