May help, May hurt. P90x is a great home fitness program i also enjoy but its not for everyone. The plyometrics routine has several high impact jumps that could cause knee pain and injuries while the leg workout has great strengthening exercises that help stabilize the knee. An exercise physiologists can evaluate your personal workout needs along with your dr to help ensure the routine is safe for you & your joints.
Consult ur doctor. P90x is meant for people that are already fit and healthy. If you have knee pain it might benefit from strengthening exercises but should be done only after your doctor clears you to do so and sometimes under the direction of a physical therapist. Be careful! overdoing it may further injure you. Check with you doctor or orthopedist first.