Some leg ext. machines are known for putting to much pressure on the back of the knee, which in turn causes knee pain. Stop doing them immediately. Start doing some detective work to find out what exactly you can do. For instance, try some barbell hack squats, Lunges, Good mornings, Dead lifts, SLDL, and so on. I don't think you're out of the leg game yet. Remember, you need to be thinking in terms of rehabilitation, so take it easy on everything, even hammies.
Are you doing any running? If not, good, don't. Try some brisk walking or some bike riding. Anything to exercise the natural motion of the knee. Also, look into getting some Glucosimine with Chondroitin(SP?). Above all, I think you need to get refered to an ortho or a P.T. They would be able to help you best.
MBBP