Can a cardio machine help prevent or at least lessen the pain in the joints due to arthritis?

Maybe. The stronger the muscles are that surround an arthritic joint, the more support the joint has and thus may experience less pain. However, improper or excessive exercise such as high impact activities can have a negative effect on the joints as well.
It might. As long as you are not pounding on the joints, exercise helps just about anything. (but be more careful with rheumatoid arthritis.) try glucosamine 500 mg 3x/ day. Make sure you have good nutrition (lots of veggies, fruits, minimum refined foods). Try same (but not if bipolar or prostate cancer) w/ b-complex. Start 400/ day, max 800 mg 2x/day: empty stomach. May take 2-3 months for full effects.