running with weak knees??

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hi,
I've got really crappy knees, they click like anything as the knee caps are slightly flat. I'm only 22 as well which is pretty lame!

the doc told me to build the muscles around the knee but avoid impact like running but eventually i want to be able to run. to run a decent mile is a huge goal for me due to a humiliation in school i want to put to bed.

any of you run with bad knees? I don't want to steam in and dislocate something but don't just want to keep walking on the treadmill!
any good tips for strengthening them?

thanks MFpals :)

Nikki