Squats tend to get a bad wrap for being knee killers, but in reality, when performing correctly, they can actually PREVENT knee pain by strengthening the supporting muscles.
What is hard on your knees is being overweight! For each pound of bodyweight your knee take 4 pounds of equivalent pressure. 2 stone overweight (12kg) is an additional 112 pounds of equivalent pressure to your knees. 4 stone overweight 224 pounds. If your knees are holding you back from exercising, focus first on losing weight.