I have heard many people recently complain about sore knees, especially if they have recently taken up exercising. Sore knees are often just the result of trigger points and tight soft tissue developing in the muscles of the thighs, which then send the pain down into the knees. A common area is when it is felt in the front of the knee, because of suddenly using your quad muscles (which are your ‘brakes’).
This is very easily sorted out with a deep tissue massage, and simple stretching after exercising will prevent this from happening again. This should not mean you have to stop exercising!