Diva, have you had a doctor take a look at your knees ? if not the first thing you need to do is get them looked at ASAP. Leg curls are hard on the knees so 5kg's sugestion of stiff leg deadlifts is a good one.
I like hamstring raises for the hamstrings
http://www.exrx.net/WeightExercises/...ringRaise.html
but that may be a little hard to begin with so a modified assisted version may work
http://www.exrx.net/WeightExercises/...RaiseSelf.html
another form of hamsting curl that may be a little easier on your knee
http://www.exrx.net/WeightExercises/...erLegCurl.html