I would say ask your doc (or physical therapist) about your specific knee issues.

For me, and my specific knee issues? Yes. I weigh over 200 lbs. Carrying that weight is super-stressful to my knee. But the leg-press and ham-curl machines? Their weight settings start at something like 45 lbs. That's a lot less than my body-weight.

The moment you have knee pain with any exercise you're doing STOP. Don't do that any more today, discuss it with someone who knows your body well (Doc, physical therapist, ec), and if you go back to it later, do it with lower weights.

That's one of the beauties of the gym machines: you can use low weights to build up strength without adding stress.

Seriously, though -- what's wrong with your knee is not going to be what's wrong with mine. (At one point I had two bad knees, and they weren't even the same as each other!) I know that my physical therapist (after proving to me that I lacked strength there) told me that 100% of his knee-pain patients had low abductor/adductor strength, and so this was one of the most important muscle groups to work. He also had me on most of the other leg machines, but always starting with very low weights. (Back then, I went to the gym regularly, and was able to increase my weights quite a bit over time.)