Hi Jess,
I am a personal trainer for over 10 years. I would suggest using the stationary bike (esp. the recumbant - legs are out in front), the elliptical is great as it is not hard on the joints as jogging on the treadmill.
Secondly, I would make sure you have a good shock absorbing pair of running shoes! These act as brake pads for you. When these wear down or do not absorb the impact, the knees and ankles take the load.
Also, think about strengthening the big muscle groups surrounding the knee joint. ie. Quads and hamstrings. This will help take the load off of the knee joint.
Remember, strength training is just as helpful in weight loss as is cardio and as we get older we lose lean muscle every year and this is what helps keep our metabolism running strong.
Finally, ice after your train, if the joint is inflamed
Nutrition wise, adding omega 3 fatty acid rich foods such as fish (salmon, sardines, tuna, bluefish) flaxseed, walnuts or take fish oil supplements will also help with any inflamation.
Hope this helps you, don't give up on moving your body and being active, there are ways around joint pain and injury!
Come check out my site
http://thinkwi.com and if you have any questions, ask away, or post a blog in the thinkwi forum.
Suzanne
