Painful knee when standing or walking.?

It depends. Knee pain can be a torn cartilage or arthritis. If it hurts getting up from a chair or going dwon the stairs, you may have patella chondromalacia, an irregularity in the surface of the back of the kneecap. For every pound someone is overweight its an extra 4 lbs. On the knee, so this can make a small problem hurt worse. To know what is going on you probably need an x-ray and possibly an mri.
Injury. If your knee hurts fopr a short time, under a month, it is probably an accute injury and will get better with rest . If longer it may be arthritis. See your personal provider for further evaluation.