Patience! Remember your weight changes with your cycles. I'm currently on the weight downslope of my cycle, but fully expect to weigh the same for the next couple of weeks as my upslope is gradually neutralized by a little weight loss. That may well be what's happening to you.
Bummer about the knees. :-( I fear for my own knees, which I think may be approaching their shelf life after 30 years of walking everywhere.

Re: Weight Watchers, the people I've known who do it have indeed done well.

And ditto dremiel. Give not into despair! Maybe try making your first priority (mentally, brainhacking!) eating healthy, with weight loss as the bonus prize.