Bad knees-Replacement exercises for lunges?

Hey All,

I am about to begin including the circuit outlined in "The Biggest Loser" book, and I would be most appreciative if those of you who truly understand muscle groups and exercise physiology would give me some advise.

This circuit includes two types of lunges, and I have bad knees as a result of my weight.

Would I be better off:

Attempting to slowly improve my flexibility and attempt to do the lunges to the best of my ability,

or,

replace the lunges with an alternate exercise?

If I should replace the lunges with an alternate exercise, does anybody have enough knowledge to recommend to me an exercise that would work the same muscle groups?

Thanks, All.

Jen